If your BGA instructor is a JAA FI with a TMG rating, you can log the flight as Pu/t and the flight would count towards your PPL licence revalidation requirements.
If your BGA instructor is an NPPL SLMG instructor (but not a JAA FI) you can still log the flight as Pu/t and it counts towards an NPPL SLMG licence.
The likelihood is that your instructor will hold a BGA MGIR in which case you log the time as P2 in your gliding logbook. You shouldn't be touching the throttle anyway so it's a gliding lesson.
Have fun. I guess you're doing a PPL to glider course, where are you going?