I just went through 5 sites for you.
Currently the best value monitor is a 22" Widescreen, which you can grab for about 200 pounds.
20" widescreen isn't worth it because they're only 30-20 pounds less, might as well go for another 2 inches. 24" would be going into the 300 pound territory apart from this Acer which hits about 250 pounds but lacks DVI input.
If you don't want to go widescreen (huge mistake) then let me know and I can dig up some more ideas.
LG and Samsung make the best monitors, I'm going to go ahead and recommend this one: LG L226WTQ, it's slightly cheaper than the Samsung equivalent and has an awesome contrast ratio.
The best price for this one is 190 from MicroDirect (decent site)
http://microdirect.co.uk/(17293)22-LG-Monitor-L226WTQ-Widescreen-TFT-DVI.aspx