When flying between countries within the european union, you have to show photo id. As Americans you have to show your passport on the last airport. If you for example fly New York-London-Barcelona, you show your passport in Barcelona. You go through security in London too but normally you don't have to show your passport there.
In my opinion, British Airways is the best airline and I never fly with anyone else.
One reason for you to change plane in England is that it's an englishspeaking country. Most people working at airports in other countries, for example France, Italy or Spain, speak english but sometimes it can be a little hard to understand!
English people are also very nice! If something happens, a small thing like a missed flight or something big like an accident, the best place to be is in England!