T3-T5 connection
Travelling on a BA ticket : BUD-LHR-BOS, with the second leg in F.
Looks like I'll be landing in T3 and connecting at T5.
1. Do I need to clear immigration at LHR at any point?
2.What is the best F lounge at T3/T5?
3. First time in BA first. Is food on board on par with the best lounge ?
Thanks!