BVB,
cheapest (not simplest)
1. Get a BGA Glider Pilot's Licence
2. BGA GPL > NPPL SLMG (well trodden route; anticipate about 20 hrs flying)
3. NPPL SLMG > NPPL SSEA (differences training only, no test)
alternative: BGA GPL > NPPL SSEA (those 20 hrs would need to be in an SSEA)
4. NPPL SSEA > LAPL(A) SEP (paperwork (+££) exercise)
5. LAPL(A) SEP > PPL(A) SEP (Details in CAP 804. Use Google to find FCL.210.A; this is the first stage that you'll need an Approved Training Organisation; anticipate 15 hrs minimum)
Principle details are in CAP804 Section 5 Part A Appendix 1.
Best advice is from the BGA.