ATCOs at civil airfields hold civil licences. I'm not sure what dispensations are available to ex- military controllers but you'd certainly have to do various licence and rating exams.
I just wonder why you are heading for the RAF, where your ATC duties would always be second to those of an RAF officer, instead of straight into NATS?
Incidentally, the 18-month initial training is just the start... there's a lot more to it and it could be several years before you become validated at an ATC unit.