Good advice from Ppdue re doing the A levels, but make sure they are in the right subjects.
You don't necessarily have to have A levels to become an ATCA, but it helps. Many non NATS units put their own staff forward for ATCO training, but the suitability tests are pretty rigorous and usually more suited to someone with a further education of the A level type.
That said, Gonzo is right - if you don't do your research properly you'll be found out at the first hurdle in any interview, especially at non NATS Airports as they rarely ask you to undertake all the NATS type tests prior to talking to you face to face.
Good luck anyway.