GATWICK!
Because, who wants to go to GATWICK!
Honestly, STN and LGW cost of living is high. On the other hand you have a good chance to become a trainer if based at London bases (majority of trainers are from London bases), you have instead of minimum 66 hours rest (after earlies) min 72 hours only at London bases. (LTN.LGW,STN)!
Another major point: which side of the road you like to drive on?
A honest answer regarding bases? It depends most on your personal preferences. The variety is tremendous! Think just about the different mentalities! British, scottish, german, french, italian, swiss!
One point is still granted: all bases have the same pay scale!
I believe most demand is for LGW. The guys from UK probably know more about that.