Although it won't reduce the flight time requirements, the knowledge and judgement you have gained will greatly help the learning process. Lessons will zoom along, only lightly touching on the parts you can already do - ensuring that you are fully able to be a glider pilot.
Unlike gaining a PPL gliding is a very hands on learning experience. The only exam you will take will be for your bronze exam, everything up to then will be done one on one with an instructor. Theory will be explained, demonstrated and then you will get to fly it.
Most importantly learning to glide won't break the bank. Rental of a K21 with an instructor will cost you only £45.60 an hour! Or you can pay a fixed price of £900 per year and only have to pay for launches.