As Chilli rightly says, you have do APCs in the ratings you will be validating at the UK unit you are going to. I believe there is also a time limit within which you have to start the training at the unit.
I did it once having been in the UAE for 6 years (have since returned to the sandpit) and given you have to perform to college standard in the APC it's quite daunting, especially for a rating you haven't used in anger for some time e.g. Approach Procedural.
But it's doable so don't let it put you off. Just keep that 4 year milestone in mind if you do plan to return. Some come here and become happy to stay long term so it becomes less of an issue as time goes on.
USA isn't an option anyway unless you have citizenship.