Admittedly progress isn't always even and some are late developers, but I would be seriously concerned that somebody who can't achieve Grade 4s at the age of 16 (with resits) wouldn't be able to access a degree course, unless it really were one of the "Micky Mouse" degrees people claim exist.
I couldn't disagree more.
At 12 I was put on a fast track to take Maths, English Language and English Literature O levels at 14. I passed the English ones with flying colours, but struggled with the Maths, and half way through the syllabus transferred to the 'normal' class. By then, the syllabuses had diverged, and I wasn't interested. This was a 1970s comprehensive, which saw achievement as getting a few top stream kids to apply for teacher training or the civil service, and assumed that girls would marry young and have babies, so didn't need many qualifications. The teachers weren't interested, I was always a rebel, and didn't see the point in trying too hard, as I planned to leave at 16.
I did leave, but did A levels (not including Maths) at college, did well, then went to university, where I didn't need the O level, and haven't looked back. I am far from innumerate - I just can't do things like quadratic equations, as I wasn't taught them properly. It has made no difference to my life.
Similarly, my daughter is dyslexic, and found English difficult, although she can very easily hold an articulate conversation, and argue her case. She passed her English GCSE, but even if she hadn't, it would have been highly unfair to stop her from going to university, where she did well, on account of having problems with writing things down.
Neither of us did so-called Mickey Mouse degrees, although I have never understood what they actually are - the phrase has always struck me as a rather unpleasant sneer by those who want to cling to the golden ticket of an education they got given but want to deny to others.
IMO this 'screening' is simply a way of reducing the numbers of working class kids getting to university, so graduate jobs are kept for those who have always seen them as a right. Middle class parents will pay for tutors (or coach their children themselves) to get them through their GCSEs - it is unlikely to stop many of them from getting a university education.