You will be required by he UK CAA to undertake an "Assesment of Prior Competence", which requires you to attend a one or two day course to demonstrate your practical, written and oral skills.
It involves, several simulator sessions, at least one oral board, and several written tests.
Depending on your performance and test scores, you will be informed on what is required in order for yo to obtain a UK CAA ATC Licence.
The absolute maximum dispensation from any course is 50% of the course time, although I would not like to join any course half way through and expect to pass it.
However, before you take any course with NATS, you will be required to formally apply for a cadetship, and undertake a further two day selction test.
Additionally, you may be asked by NATS to pay for some or all of your own course fees, with the proviso that if you pass all the courses, you will have your course fees refunded.
You will probably be expected to have to pay for all your own accomodation and subsistance fees.
In doing this, NATS are complying with all European legislation and the principle of a common European licence.
Ultimately, it comes down to the fact that NATS and any other UK ATCO employer, just as the Greek, French or any other European organisation, doesn't have to look any further than it's own back yard for potential ATCO trainees.