If you want to do Psychology in university then I highly suggest you picking Psychology in A-levels.
1. Most of the things they'll teach you in university, you'll already know, due to the fact that most of their modules relate to A-level psychology.
2. It's fun and easy to understand if you have a passion towards it.
3. There's already some biology towards psychology that you'll learn. E.g Psychopathology, Bio-psychology and the Biological Approach.
4. Some unis do require you to take psychology as an A-level in order for you to do it in University. (However, I wouldn't stress on this too much as that's only top ranking universities)
Also psychology as a whole doesn't really link to Biology in general. There's a lot of aspects like criminal behavior, forensics and even research methods that don't necessarily link with Biology as a whole. For an example, yes criminal behavior can be linked with nature (Genes/biology) but there's also the aspect of nurture (Environment)