Earlier quoted context omitted.
They recently (a few years ago) introduced A* grades for 90+/100 scores, where A was 80/100 before and so, as you note, not very discriminating. I'm not totally sure, but I think a standard offer for one of the elite universities is A*AA these days. I'm not sure I like the idea. Only having to hit 80 reduces stress quite a bit, and it gave me time to do extracurricular things like programming, and way more maths modu…
[At least when I took them a few years ago] the admissions offices get your raw results anyway - so even if it was just 'A', they could still have their own internal A* for 90+. Surprised nobody's mentioned STEP, three exams at least two of which are required for the mathematics tripos (and I think engineering at some colleges) and is significantly more likely to require preparation. I don't think any applicant would…
But as I understand it, they can't make an offer based on that, so they're already obliged to accept you by the time they get those results.