Hello Ben,
For any gliding hours to count towards a PPL, you have to get to BGA glider pilot's licence/EASA sailplane licence standard.
Once you have a BGA glider licence you get a substantial reduction in the minimum training hours required for an NPPL(SSEA) licence, but you still have to pass the NPPL skills tests. It is likely to take more than the minimum 10 hours training to reach the required standard unless you are a very experienced glider pilot, or an exceptionally talented power pilot. You can turn your BGA glider licence into an EASA sailplane licence & get up to 15 hours reduction in minimum training hours for an LAPL(A) or 10 hours reduction for a PPL(A) but you would still have to pass the skills test & there is no guarantee that you would meet the required standard in minimum hours.
The reality is that you will spend more money getting a glider licence than you will save in power instruction costs. If you enjoy gliding & would have spent the money at the club anyway, it's nice that you can save a grand or two; if you don't like gliding, it's money down the drain.