try the BHPA for a start
www.bhpa.co.uk, they have all the contacts, they'll tell you also about transferring qualifications, and insurance, many clubs won't let you on their sites without bhpa membership minimum
flying in the south east

the best site down there is devils dyke, horrendously overcrowded and best avoided though it is a good site.
i hope you're tow launching or used to crowded sites, they are few and far between there, jump on the motorway and head west or north is my advice, i fly in s.wales, more sites, the occasional crowd when competitions are on.
weekends flyable, dunno off hand , maybe 1 in 3 pure guess, but if you can get to fly mid week you can enjoy emptier sites and much more air time, i occasionally fly mid week here in wales, i have the site to myself usually
up north from london is derbyshire, the local club magazine i'm reliably informed is called 'spoilt for choice', says it all.
regards jeff farr