I do psychology atm.
Most of the time, you don't need anything as it's all taught assuming you have no prior knowledge of the course. The best combination would be Biology, Psychology and Maths/Stats, but you won't need the a level in math/statistics much because it's only very basic stuff we need, and most courses use a package that calculate the statistics for you (SPSS). So i'd say choose whatever you enjoy the most.
More essay based subjects would also pay off, sociology, english, stuff like that.