If I recall correctly...
With an FAA Licence you can fly a G-Reg Plane but only in the UK.
With an FAA Licence and an N-Reg plane you can fly anywhere in JAA Land and of course the USA.
With a JAA-PPL licence you can fly an N Reg aircraft in the USA or your home country.
Plenty of groups run N-Reg aircraft but if your planning to fly on an FAA ticket be prepared to constrain your flying to the UK if its a G-Reg group or rental.
I think the above is the rule of thumb, I am sure it will be corrected if not.