I'd recommend maths (especially if you are naturally inclined towards the subject- though definitely possibly to do amazing without being a 'mathsy' person, you just have to put in more work but you could definitely end up doing much better than others in your class who have perhaps overestimated their ability). Although not essential for medicine, a lot of Unis prefer maths/physics to psychology and are more likely to give offers. However if you are super passionate about psychology go for it as you will get a good grade if you put in the work!
Obviously there are certain subjects that are more favoured but i medicine a lot of the time it is your UCAT scores/interview. I know someone who got into Dundee for medicine (which has imo got one of the best courses in Scotland, maybe the UK for medicine) and she did Bio, Chem and Music. So it doesn't actually matter too much.