In very simple terms two of the three must be the same;
Airspace
Licence
Registration
Thus N reg, FAA Licence - any airspace
N Reg, UK JAA Licence - UK airspace
G Reg, UK JAA Licence - any airspace
G Reg, FAA Licence - UK airspace
etc.
There are, of course, exceptions - which no doubt will be pointed out in subsequent posts.....