Not read the whole thread but would like to point out that there are a couple of other options for A Levels. Kellett and Harrow - are both of which are expensive. Kellett prides itself as being academically-non-selective, but does go out of its way to offer academic support where ever it is needed.
I am also of the opinion that IB, in the context of 'average' is students who may be good at one thing but have little interest in some subjects, is a bit overrated.