Again, I'd like to thank everyone for their help but in particular ExXB. One recent concern I had is about the difference between the US and the UK/EU. We've flown together in the US and I just told the airline when booking the flights that we need assistance and they always helped, but see there's a difference between the US and UK where in the latter case it's the airport's responsibility, not the airline's.
However, having looked a little further on BAA's website, I now see they don't deal directly with assistance requests but it has to be done via the airline (who then, I hope, pass on the request to BAA). This is great because this is how I've always done it and know the process, I just hadn't done it before for a flight arriving in another country. Obviously there's an additional layer of things that could go wrong but at least it seems I just have to go through the airline and nothing else.
Well, I'm sure it'll be an interesting trip, not least because this might test the codeshare agreement between AA and BA and their relationship with BAA but again thanks for everyone's help. I also look forward to the bollocking I'll get at passport control if I get this wrong!