If it's a small consolation to you, I took my maths O'level in a comprehensive at 14 and passed (one of the youngest to still hold an actual O'level as we were the first GCSE year and I was youngest in the academic year, so it was only child geniuses who are younger).
Having said that I still maintain I would have benefitted from grammar school, as I believe I'd have been dragged along by the bootstraps of others rather than running with the crowd I did. Thankfully my family could never have afforded private education, which I am totally against (even for a Tory, yes I know!)
Two of my best mates went to grammar school and they both think it made a huge difference in their lives as neither is from wealthy families (one was from a single parent family). I was local comp but lucky it was a decent enough area. I’ve always liked the idea of grammar schools for kids from more disadvantaged backgrounds/those whose parents couldn’t afford private. It’s a massive social leveller
ps didn’t realise we had a child genius on SBT. You didn’t like antiques when you were young did you ?