Thing is to get the extra time you will need to be assessed to need the extra time and show that it is your normal way of working-which will be hard if you aren't in school.
It's not a ASD diagnosis = any specific adaption in exams.
It's showing a need of a way of working in exams to level the playing field = the adaption.
For example, my ds was diagnosed with ASD and ADHD. He was assessed as needing to type, but not extra time. His friend with the same diagnoses, was assessed as not typing nor extra time, but needing breaks during the exams.
My dd, who has no diagnosis, looking back, we should have asked for her to be assessed for extra time, and I think she'd have got it easily - she just works slowly.
I very much doubt an extra 10 minutes would be the difference between a B and an A*, and so probably you did better in the mocks because the paper suited you/the teacher marked leniently/some other factor.
I mean, I'm a quick worker in maths, and I'd reckon maybe I could have got an extra 10-15 marks in 10 minutes. I've looked at the difference between a B and an A* and it's about 90 marks. Yes that's over 3 papers, but we're looking at needing an extra 30 marks per paper. And I'd think it would be far fewer marks in an essay subject. (there's also more marks in a maths paper-they're out of 300, which very few are; most are out of 200, so if you're looking at the difference in marks and thinking it's less in history, then you need to also consider they're harder to earn)
You've also got to add in that some unis won't look at a retake as well as studying on your own will be hard, and you may not go up at all.
So I think the best thing you can do is spend this term researching which uni you like that will take you at those grades, or slightly higher. See if your teacher will do a reference which refers to the fact you were diagnosed during A-levels and you may get an offer on one of the higher ones.
And look for something to do this year, that shows your interest in the subject that can show that you're doing a useful year.