The only people who can tell you whether a current lice is valid for use in the UK - or what you will need to do to 'convert' it to a valid one - are the CAA licensing people. There might be rules that cover your situation in one of the CAA's books of rules but they were never very explicit on this topic in the past. As for whether NATS would employ someone who holds, or has held in the past, a non-UK licence is a business decision and nothing to do with licensing.