cfwme: It is a good idea to read the last 20 or so pages of this thread. held within is excellent advice covering most of the topics you will want to learn about.
MjaM: I believe NATS state somewhere on their website the required level of English, although I can't find it off hand. However I do know that NATS is approved to test people at all levels of the ICAO scale in English Language Proficiency. (reference CAA website).
Sorry, that may not be much use but, it might act as a starting point for your research?
Phil