I don't think posters realise that, if introduced, these proposals will simply codify existing de facto policy.
As pp have alluded to, for nearly ten years now it has been compulsory for students who fail to achieve a GCSE grade 4 in Maths or English Language to resit these qualifications until they pass, or turn 18 - whichever comes sooner. It is a condition of their FE funding, which also allows for them to take an equivalent Level 2 qualification if preferred.
Without 4s in English and Maths it is already the case that you cannot embark on a Level 3 qualification post-16. School sixth forms will not take you to study A Levels. Colleges will not admit you to study any Level 3 course, whether A Levels, BTECs, NVQs or whatever. Thankfully this does mean that in many FE colleges the catch-up provision has improved significantly and there is always a big push to get students through on the November resits. Many colleges will then allow the student to transfer onto Level 3 courses.
The anecdotes on this thread about very bright people who have failed maths (although isn't it interesting that it's always maths in these cases?) and then gone on to excel in higher education are inspiring but the door has been shut to those people for nearly ten years now.