Try it this way: get yourself a copy of the FISO (Flight Information Service Officer) Manual and by self study, take the exams for a FISO licence. Once you have passed the exams, try contacting local airfields to see if they will employ you as either an ATSA or a FISO (Aerodrome). If you get employed as an ATSA, the airport may just be willing to sponsor you through ATCO training given the fact you have already passed the FISO exams. If you get employed as a FISO, this will give you 'prior experience' grounds to present to NATS. It will also be an indication to yourself of whether or not you have what it takes to be an ATCO, after all it would be rather a waste of money for you to start an ATCO course then find you don't have the ability.