Being based in London, i've noticed it is considerably cheaper to fly long haul from other parts of Europe.

If i get a cheap connection from Europe that goes via London (where i live) and on the return leg simply leave the airport in London (and have no luggage) is there any flaw to this plan? Will i still get my miles/tier points for the journeys i did take?

No problem being allowed to skip the last leg. You might not get FF miles, maybe you will, depends on the airline. You surely won't get credit for the missed last leg, even though you paid for it.

You will still have to take the originating flight, which will involve some type of re-positioning.

Don't cancel in advance. They will readjust the fare if you do. There's always the odd chance that the last leg will be delayed/changed. If that happens, you will be entitled to a free change.

Excellent, thank you.

How about if i notify the airline upon landing that i won't be taking the final flight, will they appreciate that or am i more likely to get into trouble?

What you are doing is Hidden City Ticketing where A to B is more expensive then A to B to C so you get off in city B and go home.

Not sure how the UK based airlines (BA) feels about this practice but in the US the airlines are not to pleased when people do it. There is a company here in the US called Skiplagged and they show flyers how to utilize HCT. UA's reaction to this was to sue them.

HCT is against the T & C of the US based airlines and they can, theoretically, if you did it enough, suspend your FF account and take your miles. That being said, if you do it occasionally you probably will have no problem.

Do not tell the airport staff that you are not taking the last leg. They can, again theoretically, try to charge you for A to B, since that's what you flew.

Don't notify the airline, they may re-fare the ticket. Sorry to hear you had an upset stomach and were stuck in the restroom when the gate closed.

The other risk, which may or may not be high depending on the airline and routing, is in the event there is a cancellation at the start of your return journey (say, due to weather or mechanical issues). If you're booked AAA-BBB-CCC, and the AAA-BBB flight is canceled, the airline may rebook you on a connection through some other city (AAA-DDD-CCC)...all they know or care about is delivering you to CCC. In which case, you'd have to come up with a convincing story as to why you must connect at BBB or you may be out of luck.

I believe that Heathrow and maybe Gatwaick have very large airport taxes. These are low if your are transiting. If you skip the last leg and are no longer a transit passenger then the airline may come after you for the extra taxes.

Ha ha!

That is a very good point... unlikely but worth considering.

The airport taxes are charged when you leave LHR or LGW. They are not charged if you are transiting or arriving. We are actually coming back from Paris and transiting in LHR instead of flying LHR to EWR. Even taking into account the Eurostar fare we are still saving about $500. in taxes for the two of us and we get to see Paris again.

There are hundreds of threads in the BA forum about these "ex-EU" fares. The FTers there seem to do it as a matter of course.

You'd likely only have trouble if you do this for multiple flights in a fairly short time span.

The UK treasury only chisels air passengers when they leave the UK. Passengers who transfer or arrive in the UK are not taxed.

The "airport tax" being incorrectly referred to is the UK government Air Passenger Duty for departures only from all UK airports.
There is a technical difference between a tax and a duty (not that make a difference to pax)

This doesn't happen often, but it does happen. I used to fly from London to Boston via Chicago for the miles when I wasn't in a hurry. Once my LHR-ORD flight was cancelled and AA wanted to put me on their LHR-BOS nonstop. I explained why I wanted to go to O'Hare. The agent said "no problem, I'll put a note in your file to give you original routing credit." I said "fine." If I had really wanted to be in Chicago, though ...

So, be sure you have either a convincing story, as suggested, or a Plan B for what to do when you end up at CCC. After all, that's where the airline said it would take you when you bought the ticket.

There are threads on FT from people who suggest that they are in the know and have heard that BA plans to crack down.

Note that these are anonymous posters on an anonymous board quoting anonymous sources. You may credit that as much as you want.

If men in black suits with reflective sunglasses meet your inbound flight, inject you with a paralyzing substance and trundle you onto an aircraft where you wake up on the Continent, please report back.

It's odd how you seem to flap with the prevailing winds; in the BA forum where all the kool kids like to do the ex-EU hidden city stuff, you are ever so for it. When US passengers mention doing hidden city, out you come with you wrath about "dishonesty" and "stealing from the airlines." Why is that? I find it odd.

Dear OP, please just search here for Hidden City Ticketing and read the fifteen most recent threads. It's all been discussed thousands of times. Bottom line is, with no checked luggage and no intention to do this on a weekly basis, all will be fine.