Welcome to the BA forum^
1. once you arrive it is well signposted, on the floor above where you come out of customs (there are elevators)
2. If you want to use the spa treatments make if the FIRST thing you do, as they will often fill up very quickly, then go and book in for a shower or get breakfast etc (which is really rather nice IMO) Although on the 256 they may not be able to get one as the arrivals lounge closes at 2:00pm....
3. I can't definitively answer this, although I'm sure someone else can. I know this is possible when coming from BA longhaul onto BA domestic...