Maybe NATS can judge someone able to undertake the training even if his English is not perfect. Then this people will have at least one years to achieve a ICAO level 5.....
I'm training as an ATCO in Australia at the moment. I assume it's the same with NATS trainees; believe me when I say you don't want to be worrying about anything other than your course curriculum during your training, let alone trying to get your English up to level 5. Think two exams per week, and a lot of it is rote learning of stuff you've never dealt with and won't until you get into the sim. It's a foreign language to a lot of native English speakers...
If you think getting into the job is hard, wait till you start training. And I'm not saying that as an old fart. I never studied this hard when I was doing my degree...