The heading of your thread is somewhat misleading because what you really want to know is how to become an ATCO at an airfield....
The only way to guarantee working at an airfield is to try and get a position at a non-State airfield. Some will pay for their staff to undertake approved training courses but the only alternative is to pay yourself for a training course. That will cost you an arm and a leg and you will not be guaranteed a job.
If you opt for NATS, they are undoubtedly the best employer but you have to go where they send you and that's more likely to be a centre.
I wish you luck, but will not be easy.