It depends on the comp.
DD's which employs banding tests for entry appears to stream and set... although they are rather opaque about it.
Certainly her tutor group (remains the same for all 5 years) is not mixed ability and it appears the year is split in two and then set within that, In Y7-8 they set for English, Maths and Science and the rest of the subjects are taken within your tutor group (which are basically set to start with), and then in Y9-11 (3 year GCSE pathway) they set for all subjects.
Disruptive kids are not necessarily low ability either. It's more the case that setting allows children to learn at a pace suited to them and not feel like the dunce of the class, or be bored or spend half their time as an unpaid TA.
Regarding tutoring... where do you draw the line between reading with your child, exposing them to museums and educational experiences, and helping them with more complex maths? DD was never aiming at grammar, would not have been right for her, but I did give her some Bond Books... she LOVED NVR and would do it for fun. She's ND and just naturally seems to get patterns and scores very high marks.
With VAT on tutoring - not sure that's possible. We have a whole variety of them for DD (zero help these days for SEN of the SpLd variety in state) and half are not even based in the UK. The other half don't earn above the VAT threshold, Lots of 6th formers at DD's comp offer tutoring - are you going to charge them VAT?