It seems rather silly to me that you can solo with either a GP certificate
That is the main point of the NPPL, the Medics have argued that holding a Class 2 medical guarantees nothing more, and there are no statistics to prove otherwise. Once qualified you may carry up to 3 passengers on the basis of the same certificate so going solo is of no consequence.
otherwise you may find the hours you have flown with an instructor may not count towards the licence you finally decide on
There are no non JAA qualified FIs teaching on SSEA in the UK.