otherwise you may find the hours you have flown with an instructor may not count towards the licence you finally decide on
Possible, but not that likely. Because, as Whopity says:
There are no non JAA qualified FIs teaching on SSEA in the UK.
However, if someone started an FAA PPL and couldn't finish it, but had suitable evidence to show to the NPLG assessor, that training could be credited towards an NPPL - but probably not towards a JAR-FCL PPL(A) unless the CAA granted it at the time.
Thus a NPPL with only, say 10 hours of JAR-FCL FI dual instruction (the rest being FAA), would probably need more 'top up' JAR-FCL FI time towards a JAR-FCL PPL(A) than a NPPL with exclusively JAR-FCL FI dual time would.