Um I'm pretty much sure that the common procedure is to finish 4 years of high school with O levels (GCSEs) and A levels (or IB or whatever).
But certain universities may take into consideration your unusual circumstances and offer you entry. If you're in year 11 now and are trying to get into universities without finishing your A levels, forget it. (You could try Australia lol, but that's different.)
My daughter did gcse's and then 1 year of a 2 year IB course. She then went to uni in the USA. She is on a sports scholarship though. I believe this is the last year (2007 entry) where this is possible.