Hi Chris, once you have joined your first gliding club, you will be a member of the BGA. Any further clubs may have a 'country membership' for members who are already in a BGA club.
Most clubs also do a day membership scheme to allow you a flight. (this facility may be limited to the number of times you can use it in a year.)
Each club has it's own charging scheme, for instance the Midland GC at the Long Myndd, charges a unique 'one off yearly fee' which covers the cost of all your hours. Others charge seperately £6-10. for a winch launch, and about £20 per hour.
Once you have soloed, each club would require you take one check flight with an instructor before you could use their gliders.
If you can travel to a Ridge-site club, your flights are less dependant upon having a 'thermally' day, and flying is possible over the winter months, with the right sort of wind (and clothing!)
Have a look at the BGA website for the location of clubs...
British Gliding Association >> Find a Club >> UK Map