Probably from BA's point of view they would like to argue this and of course for many passengers where it doesn't make much difference if they were heading to central London (etc). There is an argument to be had on timings where if you are 2 hours late at LHR then you may have issues claiming 3 hours at LGW but generally consumers will win these sorts of arguments since the words "final destination" are fairly clear. Some routing arrangements have a "city" rule for reservation such that in many cases agents can re-route passengers easily between LHR/LGW/LCY/STN (and technically LTN). But EC261 is very clear on the transport point (Article 8.3) that the airline is responsible for paying the cost of transport.