If your original licence is still valid, it still has the same licence number, and you haven't changed address, then the FAA one is still valid, and can be used.
If not, (or you simply want to replace it) you must go through the foreign licence verification proceedures, which aren't that much hassle, but involves a trip to a FSDO in the USA (after completing some paperwork with Oklahoma first). I believe there is now somone in the UK providing this service, but only for €300.
dp