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
LASORS C1.2 In circumstances where previous flying training towards an ICAO PPL(A) (non-JAR-FCL) has been conducted but no licence has been issued, PLD will consider the crediting of such flight time towards the issue of a JAR-FCL PPL(A). In all cases, applicants must apply in writing to PLD enclosing appropriate training records and flying logbooks for the PPL training received. PLD will review the training records to establish a course of training and advise the applicant accordingly
We had this exact situation a couple of years ago and all the US training was allowed, the person at the CAA we dealt with, said that while each case was assessed on it's individual circumstances, they were not aware of any request (with FAA training) that had not been approved.