Kurosh,
As Jonathan says, the best thing to do would be to contact NATS directly. You'll be able to find contact details on their website.
If you are already an experienced ATCO and they are interested in hiring you then they may well be prepared to discuss alternative routes into the company. Obviously there will be a certain amount of requalifying to do to convert your licence to one that is valid in the UK, but it might be possible to make that conversion without going through the entire training process from scratch (which is what you will have to do if you continue through the normal application process).
If you do have to go through the usual application process then normally you'll get invited to attend one of a selection of test days which are usually no further than about a month or so in advance. Obviously this doesn't give you a lot of time to sort out visas and flights, but a conversation with NATS might enable you to book a course further into the future. If successful at stage 1 you'll get invited to the next stage, and then if successful there you will be invited to the final stage. It can take several weeks for NATS to process your results at each stage, and then another few weeks before the next available test date, so if I were you I'd either be prepared to be making quite a few trips to the UK or to come and stay for quite a prolonged period of time (once you've factored in medicals the whole process can take over 4 months). I should also point out that you will need to ensure that you have eligibility to live and work in the UK and have secured any necessary permissions required to do so
before you apply, NATS won't usually offer you any interviews until after this is all sorted.
Good luck and I hope you manage to get it all sorted
