There's a whole section on aerobatics on the BGA website:
British Gliding Association >> Info for Club & Members >> Aerobatics >> Clubs
The clubs listed are the ones that we know about and/or replied to a BGA 'get your club listed' circular. I think Shenington could be added to that list.
The link to the BAeA proficiency scheme should be:
Pilot Proficiency Scheme
Plenty of clubs have instructors who can show you the basics. Currently, instructors don't need an official rating to teach positive G figures, so any half cat. can teach loops & wingovers if they have their CFI's permission.
AFAIK, only Booker, Bristol & Glos, Lasham, London, Southdown & (possibly) Buckminster have resident BGA rated aerobatic instructors. I think Windrushers (Bicester) do aerobatic days with a visiting instructor & the Saltby-based Fox. The other clubs listed on the BGA site have instructors who will be able to teach you to fly all the Sports level (+G) figures to a high standard.