the best a-level subject is the one you have most passion and enthusiasm for, physical education could be better for you than mathematics if those rules apply.
You said you want to study abroad so a language would be really good - french, german, italian, latin, greek?
Classics, history, literature, modern languages, philosophy.
You could focus on the ancient aspects or maybe the modern aspects of religion such as it's role in society and the politics of religion in a globalised world?
What could you do with a degree in this subject and might you consider a joint degree?
Far out thoughts might be business studies as religion involves a lot of charity work so could be helpful!