You can fly a G-reg aircraft, worldwide, on any ICAO license.
VFR noncommercial.
Ref: ANO article 26.
The above may however disappear under EASA, c. 2012.
There is also a path to a full UK issued JAA PPL, based on another ICAO PPL. You sit some of the exams but not all, plus a checkride. There is an additional concession of you have something like 100hrs TT, when some of the exams fall away, but there is still the checkride. This route gives you a full standalone (not piggyback) UK/JAA PPL. The details are in LASORS.