I will be travelling through LHR on 2 separate PNRs/tickets. Unfortunately I am not flying all on *A, but on AC/ET and then BA (forcing a terminal change).
T3: AC848 arr 08:35
T5: BA730 dep 11:40

And on the next leg:
T5: BA733 arr 18:05
T3: ET710 dep 21:00

Questions:

1. Is it possible to have interline checked-in baggage and ticketing on AC=>BA or BA=>ET?
2. If interlining is NOT possible, is 3 hours enough to pass customs, get baggage, and check in again? Or should I travel with only carry-on baggage?

AC, ET & BA are part of Star Alliance so interlining should be possible. Whether it does happen is dependent on the check-in clerk.

3 hours is not long enough if you have to pass through immigration / collect luggage / etc

BA is not in Star Alliance.

BA are definitely not Star Alliance, but they do list reciprocal interline agreements with AC and ET (shows up on Google, but am on iPhone so can't easily link!)

However... Which parts of your journey are on separate tickets? If one of the connections is on separate PNRs, the airlines are under no obligation to interline your luggage, or reaccommodate you in case of delays.

Good point - however ET & AC do have agreements with BA on interlining. Which isn't going to help OP.

AC & ET are on the same PNR and the BA section is separate. Basically, LHR is a stopover on the AC/ET ticket.

So I'm worried about the "no obligation". If that's the case, do I have enough time at LHR for customs, transfers and collecting checked baggage WITHOUT interlining. It's 3 hours each way, but I've heard that LHR could be a nightmare.

It's tight.

The difficulty is in knowing exactly what will happen - there's nothing to prevent AC or BA from interlining your luggage, and there's nothing to stop them from reaccommodating you. But you're relying on an agent's goodwill to do so. So, the worst case:

Working backwards, T5 has a "conformance" cut-off of 35 minutes. Basically, unless you're clear of security by 11:05, you will not be allowed to go airside. OK, this isn't peak time, so you've got some advantage here. But let's assume it takes 15 minutes to clear security and 15 minutes to drop you bag (I'll assume you've checked in online). So that's 10:35 you need to be at Bag Drop.

T3-T5 landside transfer is by Heathrow Express train. It takes 10 minutes from T3 arrivals to the platform, the train runs every 15 minutes, and it takes about 15 minutes from Heathrow Central to T5 departures. So that's 09:55 you need to be landside at T3.

That gives you 1h20 to disembark, walk to immigration, clear immigration and collect your luggage.

If everything is bang on schedule, you can just make it. But it just takes one piece to fall out of line - delayed flight, long queues on arrival, secondary immigration, lost baggage, Heathrow Express problems, long queues at T5, and you're stranded.

This is the worst case scenario. Personally, I'm pretty risk averse wheat comes to DIY connections and will usually only do them overnight. Others are less conservative, and will leave 5 hours or so. 3 is pushing it, so I would try and figure out what your options are if you do get stranded - alternative LCC flights, insurance, etc, etc.

There's some odd advice here.

3 hours is by no means comfortable, but I certainly wouldn't say a blanket 'not enough time'.

BA's 35 minute 'conformance' point is just before you enter security, not after it, and there is generally no queue to get to it. Any hold up in security is fine, and not counted against you (though you might have to rush to the gate).

Similarly, the check-in desks close at 45 minutes, so as long as you are at a desk by 1054, you will be fine.

The 40 minute journey from T3 to T5 is a worst-case scenario. Walk a bit faster, and arrive just in time to catch a train, and it will be closer to 20 minutes. So an average of 30?

So your 1h20 window to disembark, clear immigration and collect baggage has just gone up to 1h50. On a good day, that little lot might only take 40 minutes, giving you a good hour's leeway. With that amount of leeway, you can afford a couple of things to go wrong.

Like I said, I would certainly not call it comfortable, and I would make sure I had a backup plan, but 3 hours is by no means out of the question.

If you can persuade AC to tag your bags through to the BA flight (I believe this is more likely than persuading BA to do you the same favour on the return), then so much the better. You can connect by airside bus and avoid immigration.

Similarly, if you avoid checking a bag, and can print your boarding pass online, you can do the airside transfer:

http://www.heathrowairport.com/heath...-international

I would certainly consider that, particularly if buying a new flight/ claiming on insurance is not going to work for you in the event that you miss the GVA flight.

A final option - and not one that I recommend, although I have successfully done it - would be to abandon your bag in T3 if short of time, and collect it on the return. You run the risk of the bag disappearing forever, of course...

Why not just send the checked bag by courier if that is the worry?

You might get AC to interline your bag on the way out. No chance BA will do it on the way back as it strictly only interlines to other oneworld carriers unless it's a single itinerary.

3 hours is enough time to collect and recheck your bag in LHR, but only if nothing goes wrong.

As mentioned above, if you do misconnect, you're boned.