I'm not British, but based on what you said, you seem to be conflicted between several choices - the hard sciences (biology) or social sciences (psychology) or the humanities (literature). There is a lot of great advice in this thread, but ultimately, you need to follow your heart.

Although age 20 is a late start, you will only be a year or two older than your peers. There are people still finishing university in their 30s!

Speaking as someone who has finished his university education (in a different country), I wish that I took my time and studied a subject that I was more passionate about. Anyway, I'm just sharing my mistakes with you to give you some insight. Though you can listen to parents, mentors and friends, it is up to you to find out what you really want and make your own choices, which will likely involve making tough compromises. Good luck with your decision.