I had a similar reaction from my Dad. I'm a lot older than you, I did my A Levels so far back now that it's a dim distant memory :-(
Anyway, that aside, how you can proceed will depend on your age. If you are doing your A Levels you will be between 16-18 right? The issue is when you are under 18 your parents will need to be notified and give permission for any assessment. But if you are over 18 this becomes your business and yours alone.
You might want to wait until you are 18 and then look into it, though one thing I would recommend is talking about any letters or emails that may still be sent to your parents. I had a situation where I was at school, had just turned 18 and my mother in one of her abusive tirades had kicked me out. The problem was that the school were still sending letters to her so I had to formally request that all correspondence was sent to me.
If you do have to wait until your are 18 to get a test, I suppose you may want to think about someone who could go along with you that knows about the traits you have.
I've spoken about this elsewhere on the forum, but when I was 16-17 I had received enough comments from friends etc that indicated I might be dyslexic. So I went to see the school psychologist and talked to them about it and they agreed I should be tested. They gave me a form that my parents would need to fill in. I had been talking about this with my mother every step of the way but when I brought the form home she hit the roof! She accused me of attention seeking and that everyone involved would conclude that and besides I "had already been tested" which was news to me.
I felt very ashamed of myself and it was another 10 years before I got tested and, yep, I'm definitely dyslexic. She made up me being tested before. I assume because she was indulging in the "NOT MY CHILD" behaviour others have mentioned.
The thing is, I suspect she was worried I had autistic traits too and adopted a similar policy of bullying and denial, thinking that she could shame the traits out of me.
The reason why I'm saying all of this is so perhaps you can navigate the system a bit better but also so you you don't get to my ripe old age without knowing. You have every right to decide how to proceed for yourself and a diagnosis, whatever it is will not change who you are but it may help you know yourself better. And that is a very valuable thing!