Nick,
If you check the British Gliding Assosciation website then you will find a map with all the UK clubs. Find out where your nearest is and started making some phonecalls. As far as the legalities go, a PPL is all thats required. Respective clubs however will lay down the their own minimum requirements, which usually entails some sort of gliding experience. Normally a Bronze or Silver C gliding badge will suffice but there are clubs around who will happily accept people without any gliding experience whatsoever. At our club we have roughly 10 tuggies on the rota, only half have any gliding background. As far as tug a/c are concerened, the most common are Pawnee, Super Cub, Chippie, Robin 400 and Rallye. Any time in those aircraft will be a plus.
Good luck! Gliding is very much a 'club' past time. You have to muck in and be part of the team.
Mike
p.s. a slight aside, theres also the British Parachute Assosciation you may wish to look at, assuming your looking to build hours.