I would definitely look at taking Maths. Ideally you need to focus on where you did best at GCSE and choose according to that so that you can get decent a-level grades to get into uni. Did you excel in essay-based subjects or science-based subjects for example, and then choose your a-levels around that. Also look at various universities entry requirements for architecture. There will also be several websites detailing what subjects would benefit an architecture degree. Hope this helps and good luck!