Who should I nominate to be my UCAS referee?

I am resitting privately at my old sixth form.and I am not sure who to nominate. Should I nominate my old physics teacher ( taught me a-levels for 2 years) or my head of sixth form? Like does the academic reference have to come from someone who taught you in a-levels?

Are you applying to study physics, or something different? Who do you think has a better idea of you? If you ask the head of 6th form they will probably check with your old subject teachers anyway.

I'm old fashioned, but I think it's polite to ask if someone minds being nominated as a reference.

I think my old physics teachcer would give a better reference than my head of sixth form but I do not know my physics teacher email so I was wondering if I can nominate my head of sixth form since I have contacted him and informed him already , I mean he can just ask for a reference my physics teacher and he will just write it on UCAS Im not too sure ?