I was one of the very early, if not the first, ATCO to be posted to Heathrow not having completed the usual Ministry of Aviation courses. I was employed abroad as an ATCO and sat the UK licence exams whilst on leave and I then spent a year working non-State. All this was accepted as experience in lieu of the usual education and technical requirements, etc. I just had to attend an APC RAD Course at Bournemouth before starting at Heathrow. Certainly had a few tongues wagging in the rest room behind my back!
If you simply did a training course at one of the exterior colleges you would find it very, very difficult to validate at a busy unit so go for NATS to start with..
Good luck chrisy08...