NPPL Home Page has details and ANO Schedule 8 gives licence requirements and privileges.
EASA will have a thing called LAPL (Light Aircraft Pilots Licence). It is likely that an NPPL will have conversion to LAPL but as I understand it (ie not very much) this is not yet sure and the requirements to convert are certainly not known.
The LAPL will allow single engine types up to 2000kg with no more than 4 seats,so you would be fine for P28A, C172 etc.