Clothes, laptop, iPod probably the best things. If you have DVDs and CDs then that's cool, just don't bring too many as a) you won't have much time to watch them - always something going on for you to join in with! and b) your luggage allowance for your flight. c) Most American DVDs don't work on European players as they are NTSC and Europe has Pal. The US is Region 1 and Europe is Region 2, however if you have any region 8 DVDs they work worldwide.
As I believe you will most likely be entering and leaving the UK via Heathrow or Gatwick, apply for a railcard too;
www.nationalrail.co.uk. Get a 16-25 railcard (if you're between those ages) as it can save 1/3 on fares. Your route will either be
FOR HEATHROW: Aberystwyth - Birmingham New Street, Birmingham New Street - Reading, Reading - Heathrow Terminals 1, 2, 3 & 4 OR Heathrow Terminal 5.
FOR GATWICK: Aberystwyth - Birmingham New Street, Birmingham New Street - London Euston, Take a tube (Or a couple) across London until you reach London Victoria and London Victoria has a train that will run you to Gatwick.
Here it is fairly easy to acquire plates, bowls, cutlery etc. from Matalan or Argos. They also do have shops which sell bedding stuff here too. As for your mobile, leave your US cell phone back home and get a UK mobile. It will be much much cheaper. What you can then do is get a deal with a UK company to call all your UK-based friends, and as for calling your parents and family in the states, sign up to
www.jajah.com. You create an account on there, put your phone number in. Put some credit on it and you can make calls to the US for about 1p per minute instead of £1 + which is a hell of a lot cheaper!