I can recommend Booker Gliding Club. Its 30 mins from Marylebone on the train and from there you can get a bus, walk or get a taxi to the club, which is around 2-3 miles from the town.
Its open 7 days a week, offers a full range of courses, you can book an instructor and do 3 flights in a morning or afternoon (aerotows).
The London TMA is not a problem, you can go to 3000 feet over the field, then it gets higher as you head towards Oxford. Excellent soaring, good instructors and a very good fleet, ranging from an old T21 to a Duo Discus and everything in between.