Rob, Good for you wanting to learn to fly. Excellent decision.
Firstly you can start now, yesterday was fabulous, winter can have as many good days as summer.
Secondly pick a school you like and are going to continue to go to for the year it will take you to get the PPL (you need to like the people, aircraft etc as well as the price). This may mean driving past your nearest airfield and onto another. Think also about what you want to do with the PPL - touring, aeros, it may affect where you learn.
I just remembered , there are three schools at Kemble, The Flying CLub, Skysport and Ultimate High see them all before you choose.
Someone else will have to fill you in on Bristol.
Good luck whatever you do.