Whatttt, don't listen to them. You're very smart, extremely smart, way above the average student. Cambridge and Oxford can simply be luck, right moment right timing sort of thing, I don't really understand why they think they have the justification to be like this to you? Did they go to Cambridge or Oxford and get 100% in everything or something? I mean, even if they did I'm confused as to why they're not supportive that UCL is still an amazing university and happy that you're happy and pursuing what you like. Soon you will be away from them if you do think about it though, get those A2's done and you're free! But, also I'd ask them why they expect so much of you? At the end of the day, we're all human, we make mistakes, but as long as you try your best honestly that's all that matters right?
Hope everything goes well for you and your parents be a little less judgemental, good luck!