[ig1]

The reason that Maths A-Level is used is that it measures abstract thinking ability which becomes important for more advanced CS.

What my university (Bristol) did is that the introductory units had two variants - one which assumed programming experience and one which assumed none, that way students could self-select based upon experience which is probably a better solution.

Most degrees actually have a period where they're bringing everyone up to the same level (be it Physics, Psychology or Economics) because there's a lot of variation in school level teaching and that's always going to be there unless the government starts micromanaging every school lesson.