As above really - I'd say don't worry too much as you'll do enough work when you're there. However, I DID actually do pre-reading but that was because I took a gap year and so was at some points bored out of my mind lol. Even saying that though, I only read a bit.
If you really want to do pre-reading, the best ideas I think are:
i) If you did A-level Psychology, specifically AQA-A, check out all 15 Unit 4 options. You'll have probably done 3 at the time but knowing a bit about them all gives a great broad knowledge for PSYCH101 classes.
ii) If you want to see what you'll be learning, the best textbook for your entire first year IMO is the Eysenck book "Psychology: An International Perspective". Don't have it on the night stand though, it isn't for front to back reading, just general perusing - its like over 1000 pages and could probably kill if dropped from a small height. It is awesome though for essay work, has loadsss of reference leads you can follow up on.
But seriously, if you've just finished your A-levels this year... enjoy the freedom now while you can!