Have a look
here:
NPPL holders: Deferment of the full EASA requirement to hold an EASA licence to act as pilot in command of EASA aircraft. This means that a UK national licence holder, including UK PPL and NPPL holders, may continue to fly EASA aircraft until at least April 2018. This currently limits use of UK national licences to the privileges of a LAPL holder - VFR only, private, up to 2000kg, with no more than 4 persons onboard, and no instructor privileges - thereby having no effect on the privileges of a NPPL holder;
NPPL training: Pilots currently training towards a NPPL may continue to do. It will be possible to convert an NPPL issued prior to 08th April 2018 to a Part-FCL licence;
So you can do the training, and the candidate can fly an EASA aircraft with an NPPL (SSEA).