Many psychology courses (including virtualy all of the "top" courses) expect at least one of your courses to be a STEM subject; some restrict this specifically to only maths/FM/biology/chemistry/physics, while others may include psychology or such other subjects as geology or geography. A-level Psychology is not however a requirement for any psychology degree I'm aware off. Several indicate a preference for two such STEM subjects, although usually in this case that will include psychology and other para-scientific subjects.
For the "top" courses, their degrees tend to be fairly scientific (usually along biological lines) and involve a fair amount of data analysis, so A-levels in Biology and/or Maths might be useful preparation. Although not required for admission, if you are interested in psychology it may be worth considering taking that as an option, as usually it's best to pick subjects you are likely to do best in to maximise your potential grades. Most courses will involve some essay writing though, so having an essay subject might also be a useful background - A-level Psychology should fill this criteria. At least one university (Bath) indicates they prefer applicants taking A-levels to have one numerical or analytical subject and one humanities or social science subject.
Generally speaking though, I imagine A-level Biology is probably the most directly useful for when you are studying on the degree, as all BPS accredited courses should include some biological psychology content, and it underpins a wide range of relevant topics to psychologists and the study psychology both within psychology itself and in related areas of e.g. neuroscience and behavioural ecology. This compares to the other above suggestions, since they won't assume any of the A-level Psychology content (indeed in the past older formats were generally held to be somewhat unlike degree level psychology), and it's unlikely you will do the full range of maths in A-level Maths (I doubt you'll use much if any calculus in an undergrad psychology course).
@Noodlzzz would probably be best placed to advise on the matter though! I would note the above is specifically for psychology (assumed to be BPS accredited courses), and may not apply to e.g. neuroscience or non-accredited applied psychology or psychosocial studies courses (the former of which often requires A-level Chemistry to be taken, and the latter of which tend to have fewer specific requirements or even preferences to my knowledge).