Hi.
Dedicated eBook readers e.g. Amazon's Kindle and the Sony Reader are small tablets with a hi-res black & white 'e-ink' screen which gives decent contrast and very good battery life. Overall, the devices are pretty clunky when compared to LCD screens on say, an iPod Touch, but they are generally a comfortable size and weight to hold and carry, and some (Kindle) have other basic functions as well e.g. basic web browsing.
So short answer - buy a Kindle 3 from Amazon. Honestly, nothing else you can buy new in the UK can touch it on price/spec and Amazon have by far the best range of eBooks available. You download books directly to the device, so there's no messing about with computers - unless you want to, of course. Your only decision is between a WiFi-only model and WiFi with 3G - with the 3G version you can buy books wherever there's a 3G signal, with WiFi you have to be logged onto a WiFi network.
The only downside is that the books are in Amazon's Kindle format - you can't read them on other devices. To be fair, this is the same with other readers as well, and whilst the alternative ePub format is becoming an open standard for 'everyone else' most companies apply their own DRM scheme (Digital Rights Management - supposedly prevents copying) which makes the format incompatible between many devices.
Other dedicated readers are available e.g. Sony from Waterstones and WHSmiths, but I found the overall book purchasing experience (and the device itself) to be far more clunky than with Amazon. Be aware as well that the £99 Sony Readers from WHS etc... only have a 5" screen, and is a soon-to-be-discontinued model.
HTH, Pete.
p.s. yes, this was the short (and simplified) answer!