According to
Welcome to the British Gliding Association you would need a minimum of 10 hours instruction before taking the flying test to convert to a NPPL. You would need to pass the current JAR PPL exams also.
The requirements to upgrade to a JAR PPL (from LASORS)
The holder of a NPPL with SSEA aircraft rating wishing to
obtain a JAR-FCL PPL(A) shall have completed on SEP
Aeroplanes:
a. At least 45 hours flight time, of which at least 35
hours must have been as a pilot of SEP aeroplanes.
A maximum of 5 hours may be completed in an
approved FNPT or flight simulator. This flight time
must include:
1. 20 hours dual instruction in SEP aeroplanes
with a JAR qualified instructor, which must
include instruction in those elements from the
syllabus of flight instruction for the JAR-FCL
PPL(A) not found in the syllabus of flight
instruction for the NPPL(A) rated for SSEA.
2. 10 hours solo flight time on SEP aeroplanes
which must include 5 hours solo cross country
flight time including one cross country flight of
at least 270km (150nm), during which full stop
landings at two different aerodromes different
from the aerodrome of departure shall be
made as per JAR-FCL 1.125.
b. Have passed the JAR-FCL PPL(A) Theoretical
Knowledge Examinations in all subjects (whch you would have done anyway)
Plus passing the skill test and class II medical
So overall the cost reduction might not be that great but it would be a progressive path from gliding to a full JAA PPL.
Just an option that's all.