LTN is the main -700 base, with very little if any -300's based there. LGW has a lot of -700's, but they are re-equipping with the A319. The next place to look would be LPL, they have a mixed fleet up there but I think the majority is mainly -700's.
EDI has a mixed fleet, and thats about it.
Rumour has it that EMA will convert to the 700, but that ones been doing the round for some time and nothing has happened yet.
STN, NCL, BRS, GLA etc are exclusively 300 bases (but I stand to be corrected).
When you join you will probably do your sim / line training on both types, regardless of where they base you.
Hope your ready to have large oranges shoved up your **** by our "management". Enjoy.
P.S ORY is also a -700 base, but you probably won't get a look in there.