Perhaps it's not a bad thing to create the equivalent of an 11+ at A Level. In many ways this has always existed because there have always been requirements to A Level, so those who have not been able to take them.
I think it's right that only those suited to A Level study and who have a reasonable chance to get decent grades should do them. GCSE is a fair way to judge this. The thing is that a decent proportion of children are not suited to this kind of academic study.....just wanting most children to do A Levels and go to Uni doesn't make it possible for most.
The key thing then,N with compulsory education 16-18 is that there is actually something decent and worthwhile for those not doing A Levels. Perhaps the problem is that other qualifications are still not always well regarded so kids and their parents want A Levels when they are not suitable.
It's pretty usual for selective schools to ask for As (or equivalent) at GCSE for study at A Level in that subject. Where it's a new subject or where the subject hasn't been studied at GCSE, then they look at a similar skills subject. It is not common, but not totally unusual for kids to take A Level History or gEography without having done GCSE - but if they have an A in English and another humanity and have a good range of high level GCSEs they should be fine.
It seems that non-selective schools, which obviously actually are selective at A Level, usually take a 6 and at a push and if desperate for bums on seats for funding will take a 5. Those with a 5 have a high chance of failing or achieving an E grade.
I think people get uptight about this in non-selective schools because they somehow think a Comprehensive ethos should apply that everyone is welcome. Probably everyone is welcome onto some kind of course, but A Levels just aren't suitable for all.
Every year in August there are parents bemoaning the fact that their child with an equivalent C or a B at Maths isn't being allowed to do A Level, when the schools have made their entry requirements clear and explained them to kids and parents.
7s maybe on the high side, but are acceptable for high performing schools which want the kids to be able to progress at a fast pace which is hindered by having lower ability children. 6s seem reasonable to me for schools which have a broader intake lower down and which accept and make clear that there will be a broader range of results and final destinations and that the pace of lessons will be different because of this range.