Go to
www.caa.co.uk and search for ATSIN No 38, this explains it.
Despite the new regulations, I doubt whether NATS, the UK's main ATS provider, would let anyone from another ATS provider, let alone another European state commence on the job training at a unit without at least a short spell of assessment at the college first. And don't forget that NATS controllers are "mobile" which means you could end up anywhere between Aberdeen and Southampton. Unless you work at Athens of course, in which case you can expect to be sent to Heathrow or the en route centre on the south coast. Who would swap the Mediterranean for Hounslow? Also you can say goodbye to at least one of your ratings. Company policy is rapidly separating aerodrome units from their approach and area colleagues.
Best advice is to phone up and ask!