What PPL routes are available with gliding experience?
I have a partial answer after more research and some help on pprune.

A Bronze + XC can be used to get a gliding licence, which under the transitional arrangements can be used to obtain a LAPL(S).

In theory the route from there to a LAPL(S)+TMG  and on to a LAPL(A) TMG is feasible, however the BGA is not an ATO yet and there are very few instructors or examiners around.  This route may be problematic at the moment, but should become easier as time goes on.  There's no reason not to claim the LAPL(A) on the way to a PPL(A) if taking this route.

An alternative route is via an NPPL under the transitional arrangements.  After gaining the gliding certificate, a minimum of 10 hours under instruction is needed until reaching the standard for an NPPL SMLG.  After short conversion training, this can then be converted to an NPPL SSEA.  After 6 hours PIC this lot in turn can be traded in for a LAPL(A) SEP+TMG.  Complicated, but overall less hours and my much can be done at a BGA club with a motorglider and instructor.

Regarding the straight PPL(A) route with credit, given the statement 


  If you hold a pilot’s licence for another type of aircraft, except
  balloons, you will be credited with 10% of your total flight time as
  PIC up to a maximum of 10 hours


it would seem clear that at that some form of licence is needed, and given the gliding licence qualifies for a LAPL(S) anyway, which one is moot.