There is no problem with you exercising the full privileges of your JAR licence in the N-reg, so long as you remain in UK airspace. This even goes for the IFR privileges associated with you IR.
Once outside the UK, you will require an FAA licence in order to be able to operate the aircraft. This is because the FAA will accept either an FAA ticket, or one *issued by* the country in which you are flying. JAR licences are taken to be issued by the country in which they are...erm... issued and are not valid across JAR land for the purposes of the FAA rules.
The FAA remains the ultimate authority on who may operate an N-registered aircraft.
The oft-quoted but non-existent "two out of three rule" is rubbish and should immediately be ignored. Even as a guideline, it is wrong as often as it is right and does not correspond to any actual legislation.
2D