Originally Posted by
Deltican
Flying at the end of June from DFW-LHR on AA.. have a long layover at LHR and then catch a BA flight from LHR to IST...
Depending how long your layover in London is you'll need to decide whether to actually enter the U.K. or not
The AA flight arrives at T3 and the BA flight leaves from T5. If you decide not to enter the U.K. then you can do an airside transfer using the free bus between the terminals. When you reach T5 you still have to go through security, which can be a bit of a zoo there at times
If you decide to enter the UK, then you can get the express train to Paddington station in central London, or the slower but cheaper tube train