Where is this NPPL website?
NPPL Home Page
Q Can I fly my 'N' registered aircraft on my NPPL?
A No. This is because the NPPL is a sub-ICAO licence, and the usual reciprocity with the USA and other ICAO contracting states does not apply. As the holder of a UK NPPL, you may only fly in UK airspace in a UK registered aircraft.
I believe this is based on their interpretation of the FARs.