No, a NPPL (SSEA) holder may not
fly foreign-registered aircraft.
From LASORS 2010:
The NPPL is a sub-ICAO licence* and therefore is restricted for use in G-registered aircraft within UK airspace. For flights outside of the UK please refer to Schedule 7, Part A, Section 3 of the Air Navigation Order or Section A, Appendix F of LASORS.
An NPPL holder with SSEA ratings can now fly in France following an agreement with the French DGAC, provided the holder is in possession of an SSEA rating, Class 2 Medical Certificate and a certificate confirming a minimum level of experience. For further details of the experience requirements and cost refer to NPLG Ltd.
* Holders of a SLMG rating may operate UK registered SLMGs outside the UK providing they obtain a JAR-Class 1 or 2 medical certificate and demonstrate the ICAO requirement of 40 hours training. Application is then made to PLD for an appropriate licence endorsement.
The NPPL(SSEA) may now be used in airspace of the UK, IoM, CI, Ireland and France. However, CI, Ireland and France have individual additional compliance requirements which may be viewed on the NPPL website.
Hence CAP804 makes it rather clearer:
The holder of a UK NPPL(A) with an SSEA/SLMG rating may exercise the privileges of the licence to fly non-EASA SSEA or SLMGs, as applicable, registered in the UK in UK airspace. Use of the licence within the airspace of other countries is subject to the agreement of the relevant authorities in those countries.
The licence is also valid for EASA SSEA and SLMGs until 8 April 2015.