You can fly a G-reg, worldwide VFR, on any ICAO PPL, CPL or ATPL.
This is in the ANO and has been there for many years.
This "automatic validation of foreign papers" is scheduled to end, I gather, under EASA FCL, sometime in 2012.
Not many people know about this; had they known, the UK PPL training business would have been decimated because training in the USA is so much cheaper and if you don't have to go to one of the 6 or 7 JAA-PPL-capable schools there, you have a huge choice.
Whether you can fly a G-reg with an ICAO PPL but a UK medical I don't know. I doubt this has ever been tested. You would need to get a view from the CAA - and good luck with that because nowadays extracting answers to complicated questions like that is like pulling teeth. I've had several answers in 2011 which were written by somebody who clearly didn't understand the issue at all.
One could argue that it is OK, because the CAA can hardly say that the foreign medical is more "important" than its own one
Especially if the CAA medical is a Class 1
OTOH one could argue that, without a
matching (i.e. FAA) medical, your FAA license(s) cannot be used for any flight, which is also true