So 100 hours of NPPL(M) time will give you 10 hours towards PPL(A) - not a good conversion rate for your money spent, unless your NPPL(M) hours are almost free.
Rip off springs to mind!
Here's what you do:-
You go to America.
AFAIK The FAA will recognise your Microlight hours towards the issuance of a FAA certificate. You do a few dual / solo flights, make sure you can a) fly, and b) meet all of the remaining FAA requirements (like night flight / cross country / instructor time etc....), sit the ground exams and flight test and now you hold an ICAO PPL. You do not need to convert this to fly in the UK unless you want to, and depending on the number of hours total time you hold, conversion is a piece of cake...
Thats what I would do anyway.