Good information from holdmetight, and I can confirm that the ICAO English test used for cadets doesn't require aviation knowledge or radiotelephony. These skills would be tested in other ICAO English tests, but are not relevant to cadet entry pilots.
However you would probably find an aviation specific English course more interesting (and certainly a lot better for your motivation) than a general English course. The CX ICAO test uses the ICAO descriptors - pronunciation, structure, vocabulary, comprehension, fluency and interaction which aren't tested individually in other English tests. You will need to score a minimum of level 4 in each of these skills. If you score level 4 for each skill apart from pronunciation eg level 3, you would get a level 3 score.
Officially airlines need level 4 scores to be ICAO compliant, but I had a long chat with a nice lady in Cathay HR and she said they want cadets to have level 5. I've known retest candidates to be accepted with level 4 scores though.
Either way, aim for level 5 because you will need strong communication skills throughout the selection process and you don't want to have "borderline" level 4 scores which could slip to level 3 if you don't practice. Level 4 is the bare minimum anyway and many people (myself included) would say that it should be raised to level 5. It would be very difficult for some countries to meet the March 2011 deadline for ICAO compliance - so officially it's level 4.
I would add is that you should start improving your English early - even before you make an application. Some students have come to us a week before their ICAO test and expected a "magic pill" that will enable them to jump to level 5 overnight. If you are already at level 4, then achieving level 5 won't take too long, but the difference between ICAO level 3 and ICAO level 4 is massive. Depending on where you are on the ICAO scale, going from level 3 to 4 could take up to 200 hours.
If you are not sure of your current level, we offer a placement test that will give you a lot of insight into which areas you should improve. I recommend this to every non-native speaker applying to the cadet programme, as it can save you time and money.
This article on our blog should be helpful in developing learning strategies. You will find that the "educational environment" in Hong Kong is very much focused on passing tests - rather than developing actual functional competence in English. The way that you prepare yourself to learn is very important.
English learning strategies for pilots and air traffic controllers English for pilots, ATCs, cabin crew and airport staff in Asia
Feel free to post a comment or question on the blog too.