Hi NTFM, as a couple the window pair won't be any worse than the middle pair for leg room, just for space between you and your neighbour (who is your OH so this should be no problem )

The leg spacing is tight for a several hour trip and bulkhead is definitely nicer if you don't want the seat in front to be reclined in your face. I had this coming back from IST for the whole flight including meals which actually made it hard to eat and get my drink, etc...so my advice would be window seat is fine, just make sure you get bulkhead.

As to fast track immigration, in May, we were handed the fast track vouchers after we had descended the stairs by ground staff there between the stairs and the buses. At first I thought it was just some advertising or such but took one anyway and then realised they were for fast track. Definitely helps.

We had a great crew outbound ^ , coming back, well, let's just say I was glad it wasn't my first time on BA, it would have been my last.

Maybe worth mentioning is that there is a screening done at the airport entrance so before you get to check in or security you'll need to do the whole screening process just to get into the airport.

BA only open the check in counters or ticket desks 2 hours before so definitely use mobile BP if you have it or print one before going to the airport or you'll be waiting landside in a rather less enticing airport hall for quite some time.

The lounge is actually okay in my opinion, as long as you get there early to get a seat. An hour before the flight it becomes far too crowded and is a free for all so best to leave at that time IMO. Food was good and the service at the bar was also quite good.

Hope this is of use.