Can anyone give me a sensible reasoning why there should be a cut-off at security at all rather than a cut-off at the gate when the doors are closed?

Reasons I have seen are:

Passengers might delay flight - don't let passenger board at gate, no reason to stop him at security

APIS not done, no visa, seat not ready - don't let passenger board at gate, no reason to stop him at security

Checked luggage can't make it anymore - should not have been accepted and bag drop in the first place.

You will still take quite a lot of time from security to gate - I can run. Or: Gate might be close by. Thank you very much, I'll take my chances.

You wouldn't be able to make your duty free shopping anymore - true

Good, the fewer of those the better.

I've never missed a flight, or been through security <50mins before departure (more typically 1h20) or given a DYKWIA tantrum. However, I fall into the neurotic band where such a policy just adds unecessary stress to a journey (i.e. the tube's stopped, I've given myself 10 mins to get the earlier HEX, but might miss it now, leaving me only 15 mins to get from the HEX to security, if it's on time).

Any extra stage in the Early Rise - Taxi/Tube - HEX - Security - Plane - Car Hire - 2 hr Drive, is un-welcome. The consequence is the Miles + TP don't seem so alluring and I go elsewhere. It's really that simple.

If you go back to the OP's quoted material, that was the whole concern. BA start to lose customers from this policy.

That sort of attitude used to be reserved for the likes of Mr O'Leary (with the odd f-word thrown in).

Again: nobody minds a T-(whatever is reasonable) cut-off time at the gate. If you're late, you're late, and you'll miss the flight. If you miss the deadline for check-in luggage, you go without your bags, or not at all. Fair enough. Arriving at LHR T5 at T-30 on a domestic or EU shorthaul, with PYOBP and a UK or EU passport, only to be automatically offloaded from the flight, is in quite a different league. And blaming BA's LHR delays on late PAX really takes the biscuit.

As you rightly conclude, no.
Believe me, if I thought this were the case I'd be all in favour of the new rule. I'm normally at the terminal at least 90 minutes before departure. But, as you say, it is nonsense. I can assure you I'm not part of the DYKWIA crowd, and I can't see that anyone is (no one is suggesting the plane should be delayed for them) and I have never once been one of the last on the plane, stowing my luggage as everyone else is all ready to go.

Furthemore, many destinations don't need visas. Domestic ones don't even need passports. Only a few need api and this is done in advance via mmb (you can't even check-in online if it isn't done), as is seat assignment.
All a bunch of red herrings. At best, a rule for the convenience of the airline, not the passenger, although it is hard to see how this is really the case. Whatever all these mysterious things are that really need to be done by the airline between security (note, not check-in) and the gate, other airlines seem to be able to do them in five minutes. BA is behaving like the NHS of airlines: sit around all morning waiting for your appointment so we can see you when we feel like it.

Both are part of the "lowest common denominator" argument. Some people might need 35 minutes, so let's make everyone need 35 minutes.
Seems to be the only thing that makes any sense.

The T-35 cutoff is not just there so that people are in the right place at the right time. It is to check that everyone is "ready to fly". At T5 there are zero transactions at the gate, they scan you boarding pass, you get on, that's it. If someone didn't have APIS data in their booking or incorrect data, this can lead to delays with boarding. Also, Visa checks are done landside at a FBD if you're with hold luggage or at a visa desk if you're hand baggage only. Once your visa is checked, an indicator is added to your booking to say that it has been checked. Passengers requiring visas for other destinations that are not part of the visa check are limited and so this is done at the gate. This again removes the transaction from the boarding process.

The theory was also that there are no seat changes airside, once you're checked in, that's your seat. I don't know how that airside customer service desks/lounges are handling this and may still be changing seats. Though I know that seat changes will not be done at the gate.

So you see, it isn't just so that people are physically on time for their flight, it is also to remove risk of a delayed flight by removing complexity from the boarding process.

If I had 500 BA Miles for every person who posts on here that they are never flying with BA again because <insert personal gripe>, then I'd be a BA Miles millionaire.

As a shareholder, I am more than confident that BA will lose very little business because of T-35. And what it does lose will be more than made up if T-35 means that BA achieves a punctuality record of which, for once, it can be proud. Perhaps then some of those who have flounced off to A. N. Other carrier that allows them to rock up at T-10 (only to be delayed while some visa crisis is sorted at the gate) might return.

xxxxx has given the reasons why T-35 is a necessary part of the punctuality drive. There will always be those conspiracy theorists who believe that it is really a plot to get people to spend more time shopping (just as there are apparently some people who believe that Diana was murdered by the Duke of Edinburgh), but you can't base a sensible argument around such ramblings.

Punctuality is good. So we can blame the design of T5 by not having holding pens at the gate and therefore requiring T-35 at security

It's unfortunate that pushback seems to be the measure of punctuality. It would be more useful if punctuality was measured by "wheels-up" times, thus allowing management to focus on all the other causes of missed punctuality ("where's the tug driver", "APU's not working, waiting for some jump leads", "waiting 30 minutes for a slot even though we pushed back at T-5", or "waiting to cross active runway" etc.) which are conveniently ignored.

If the objective is to claim X % punctual "departures", then sure, pushback is a good measure. If you're trying to get your passengers to their destinations on time, then wheels-up would be a much better one.

Agree absolutely. Made the same point in the thread on punctuality.

Thanks for this explanation, but I still don't get it. If the philosophy is no transactions after security, then don't let passengers through security that have a visa or passport or seat or ticket problem.

Why stop people that can still make it to the gate at T-15, or even at departure time? The flight might be delayed and the passenger might make it. Loadsheet calculation etc have to be made after the gate closes anyway. Just set the correct cut-off for gate closing, and everyone should be happy,

My own thoughts are that this is a very good idea and consider it to be perfectly reasonable. I fail to understand how anybody can honestly think they will be able to arrive at the gate with suffcient time to allow for the completion of boarding the entire aircraft and for all other processes that go unseen to be finished, if the passenger cannot even deign to be at security 35 minutes prior to scheduled departure. We are talking about planes here, not trains.

Nobody is that important anyway (and if are, they aren't flying through T5). In any case, important or not, planning one's diary in a way that allows less than 35 minutes to get from ground transport to gate, especially at an airport as massive as LHR, is completely naïve and unworkable.

There has to be a boundary set, and it has to be adhered to. T5 is a big place, so 35 minutes seems a good measure. You can't make exceptions for people without hold baggage, for frequent travelers, for Goldcard holders. It has to be kept simple and easy to understand. Making exceptions or using experienced travelers as the standard only will complicate an otherwise simple system.

I also question how much of a problem this will really be... I don't believe there are so many passengers needing to arrive at the airport less than 35 minutes that should they be refused boarding, BA would feel pain in their bottom line.

This is a good explanation of why the T-35 rule exists, thanks. ^ Clearly, early checking of the APIS and VISAs are a progressive step forward.

Sounds reasonable, there will always be adjustment for passenger's who don't make it for any given reason. It would be easy to flag someone who was late through security, but with all necessary info in place as a risk for not getting the flight?

Perhaps you're taking my personal example too literally. I'm not, as you suggest in your earlier post going to stop flying BA because I don't like the T-35 rule, it's just going to make it less appealing (also the extra 5-10 mins on the HEX to T5 doesn't help).

Choice of carriers will naturally depend on a whole host of factors including price, timings, routing, rewards, quality, etc etc. I hope as a shareholder you would want me to have BA as a preferred carrier for which I am willing to pay a premium?

I hope puntuality does improve. I hope that the people at T5 security will let me through at T-34 on the one occasion in a hundred when despite all planning and contingencies I arrive a minute past the cut off for my short flight to MXP (not in a DYKWIA way, in a "darn I'm having a bad day way, and wouldn't it be nice to get where I'm going on time"). I just think it will add extra, and probably unecessary, stress to a bad journey.

Ho hum. Just watch what happens if John Mack turns up at T5 20 minutes before his flight.
There are always exceptions.

Nobody plans to be at an airport 35 minutes before departure - I plan to be their 1 hour in advance, and maybe 1 1/2 hours at LHR. But then, there is the day where my meeting runs 10 minutes longer than planned, the taxi gets stuck in traffic and I just miss the HEX, and there you go - without planning it, I am at security only 30 minutes before departure. I cannot afford to build in even more buffer than that, as it would make day trips quite infeasible.

Now why should I automatically be offloaded? I don't need a visa and I have printed my online BP with a seat number. Usually gates will close probably 10-15 minutes before departure. So, unless my plane is on the gate furthest away in T5B, I have a high chance of making the flight, without holding anyone up.

Not if a passenger turns up 20 minutes before departure there isn't. It's nigh on impossible for someone to get from check in to a gate at T5 in that time.

And where have you got the idea that flights have been moved back from T5 to T4? The date for long haul flights to move to T5 from T4 has been delayed but no flights have been moved back from T5 to anywhere at LHR

As you say you've not been to T5 - why do you find it so hard to believe those people who have?