As I understand it to go from a NPPL to a UK JAR PPL, it used to be all of the instruction had to be done at the NPPL phase by a JAR instructor. It may have changed.

If you are after a CPL, far less hassle to do direct for a ICAO PPL be that JAA, FAA or wherever. Shortcuts are often met with complications later.

You can only fly within the UK FIR with a NPPL.