Very best of luck OP, but to get the grades you really want to get, and get yourself into UCL, you literally need to work your arse off. Absolutely annihilate past papers. Make sure you get 100% in all past papers, especially in Maths papers, because Maths is very systematic. Read through the syllabus for the sciences, and also use different books other than the normal textbooks you use for Maths A-Level. I'm currently using "Core Maths for Advanced Level by L. Bostock" to help me try and get the A*.
I'm saying this out of experience. I got BCC in my AS Levels. Had an Open Day interview at UCL for Chemistry, but I was unsuccessful like you. So I worked my arse off last year and I achieved AABa in Maths, Chemistry, Biology and AS Further Maths.
I'm currently re-taking my Maths, Biology A-Levels as well as taking Further Maths A-Level to get A* for Maths and Further Maths and hopefully get the A for Biology. I re-applied to top universities this year to 4 mathematics schools and one Natural Sciences, and I got 4/5 offers from my universities. I'm currently an offer holder from UCL for Natural Sciences
I need to get A*A in any of Maths, FM or Biology.
If your heart is set on getting into UCL, then bloody work your arse off. Honestly, I need to say that to myself because I'm nowhere near as hard-working as I was last year.