Check out the Samsung 205BW, a 20.1" widescreen. Don't know how much it costs in the UK, but im pretty sure its around 350 pounds, if not less. As for response time, smaller is better, this one has 6ms which is mooooore than sufficient, tft technology has increased dramatically pushing prices down and anything around 16ms is more than enough for the regular home user.
I prefer samsung monitors, but there's plenty of fish in the sea:
Asus MW201U (2ms response time)
BenQ FP202W (cheapest)
LG Electronics L204WT-SF (most for the money)
These are all 20.1" widescreen TFT monitors.
I know this because I'm trying to make up my mind for which one I should buy. I'm sorry I can't give you any details about resolution, you should probably check on that before you buy.