There's no such thing as an official glider pilot's licence in the UK and no minimum qualification hours, so there's no offset for PPL (or CPL!) holders.
The sequence would have to be to turn up at your local gliding club, do some training & go solo (hopefully fairly quickly if you've got a PPL). You would have to learn what rudder pedals are for, winch launching or aerotowing, launch failures, how to share thermals safely with other gliders and field landings
)
You would then have to do the Bronze C theory (shouldn't be a problem for a PPL holder) & bronze qualifying flights, then get signed off for cross country before setting off into the wide blue yonder.
