Hi Charlie - guess i'll see you there!
Ok here goes:
as an british citizen you automatically get the level of 'competent' however that doesn't get you any Visa points. If you score at least a level 7 in all four IELTS categories you get classed as 'proficient' and get 10 points towards your visa app - well done. If however, you get at least a level 8 in all categories then you are classed as 'superior' and get a whopping 20 points towards your visa application - clever boy!
Hope this helps!