Alphamatt.. As you have seen,. you do not have to join NATS to be a controllers, but NATS is the best employer of professional controllers in the UK in terms of salary, working conditions, time off, pensions, etc. The alternative is to obtain a position at a non-State airfield (i.e. where ATC is not provided by NATS) and see if the employer will subsidise your training. All Air Traffic Controllers providing ATC services at licenced ATC units have to hold ATC Licences, which equate to pilot's licences in terms of training, medical requirements, etc. Make no mistake, training is hard graft and takes a long time... and if you have to pay for it yourself it'll cost you an arm and a leg.
From the questions you have asked on a couple of threads I suspect that you are not overly familiar with the requirements for being an Air Traffic Controller. Licencing requirements are available here:
Take a look; it will provide you with heaps of information.
More information is in Part B of this document: