Thomsonfly's 757/767 bases are Gla, Ncl, Man, Ema, Bhx, Stn, Ltn, Lgw, Brs, Cwl. 737 only bases are Cvt, Dsa and Boh.
The best base? That depends what you want and where you want to live. If long-haul is your preference then Lgw and Man are the bases to choose between. If you're after a DEC position then it's the 737 and don't have any delusions of quickly hopping across to the 757/767 fleet since as PP says, you'll be joining at the bottom of the seniority list with many Captains ahead of you.