Hello and welcome!!!
You have all the rights to be excited and congratulations on your new job!!!
To answer your question, all the Customer Service training and Medical will be done at Hangar 6, LGW.
For SEP you'll be at Cranebank with overnight accomodation paid by BA at the Arora Park Hotel. There's a BA bus (though not always on time and not always shows up) that links the hotel to Cranebank. I think it is the number 7.
The Arora Park hotel is outside the busy area of LHR, and has a chinese restaurant at the ground flor. Reasonable prices and good quality. Opposite the hotel there's a pub and an Italian restaurant (their pasta is yummy).
Anything I can help with please let me know
Ciao
FBW
*** Love at first Flight ***