First of all, you need to get a glider pilot's licence, so Bronze + xc endorsement & then paperwork & cheque to the BGA who will then ask the CAA to issue you an SPL.
The amount of crediting you get depends on whether you do an LAPL or PPL course.
Part FCL has all the details, Annex I FCL.110.A(c) for the LAPL, FCL.210.A(d) for the PPL. Basically, up to 7.5 hours for the LAPL, up to 10 hours for the PPL.
CAP 804 is long out of date.