Taking A-Levels and Going to an Overseas University on a Scholarship

I go to a Girl’s Catholic School in SA (South Africa) and maintain an above 90 percent aggregate. I am terrible at sport, but I am involved in every cultural activity my school offers. I take drama classes and French outside of school, and I took a Philosophy course in 8th grade, I am currently in Grade 9.
I am thinking of taking an A-Level online, in any subject of choice: History (my favourite at school), Philosophy (an interest of mine not offered) or Sociology. I am aware the workload will be a lot, entering my grade 10 year. I am taking Physics, Biology, History, English, Math, and Afrikaans at school, not A-levels obviously. So, my question is… Which A-Levlel? Is it worth it? And how am I going to successfully apply to a UK university? Am I Oxford material? Any other suggestions to help me get to that standard?
Much appreciated
Goose (Isabella)