"At what point does it become acceptable for comprehensive schools to select on the basis of ability?"
ATM, education is only compulsory until Y11. Education is not compulsory for sixth form: the kids don't have to go and, conversely, the schools don't have to accept them. Simples.
As and A2 are very different to GCSE, it's not more-of-the-same-but-a-little-bit-harder, so it's only to be expected that schools will wish to have those students who might succeed. And it's only fair on the students to warn them that they might be about to waste a year of their life doing the wrong course.
Are you going to suggest comprehensive Universities next?
"I'm not sure that 8 at minimum of B isn't more difficult than 6 at A."
The 6 at A/A* Grade is just the minimum to get you on the interview list, it is not an automatic entry. Most get way above this. It's the equivalent to thinking that AAA at Level will get you into Oxbridge: it won't, it is merely the first rung on the ladder.