YWG ATC
I have experience of entering NATS through the APC route, converting from military to a civilian licence. I suspect the APC route you would follow would be the same:
If you enter NATS through the APC route to convert your licences, you will be assessed over 3 days (each) in aerodrome and radar. If you are deemed to be of a sufficient standard (as an current controller there should be no problems), you then carry out aproximately 3 months training (each) on aerodrome and approach radar. NATS pay you as a student as you go through this shortened course, and you negotiate where you will finally be posted to before you start.
Speak directly to NATS Airports Personnel Dept (020 8745 3721) - they're a friendly bunch and will advise you better. Good luck.