The 430ex ii is probably your best option (it's what I hope to get for Christmas
). Canon only makes 3 flash units the 270ex (£100), 430ex ii (£200) and 580ex ii (£325).
This is really narrowed down to the 430 ex ii and 580ex ii because the 270ex ii is fixed and can't be swivelled or tilted (it is basically a slightly more powerful version of the built in flash and doesn't have many features). The 580ex ii is more powerful, has a slightly faster recycle time and can swivel 360 degrees. But it is heavier, bigger and much more expensive. The 430ex ii will do everything you need (unless you want to use it as a master flash) and is a good choice for a beginner.