As a Swede English is not my naitive language (even though I am fluent).
What you need is an ICAO level 5, NATS will test you for this if you make it through the application process. If you ahve doubts about being able to achive a class 5 maybe you should ask HR to test you beforehand if possible, it may be though that their reasoning is that if you make it thruogh the entire process and interveiw (which is all in English conducted by naitive speakers) you'll manage a class 5.