The only 20" laptop i know of is the dell xps m2010. Which has a 20.1" panel on it, isnt much of a laptop though you can move it around! It's resolution however is 1680x1050. Thats pretty much as high as you're going to get, anymore more and you'll be into the dell 24" monitor territory.
I'd personally say go for a laptop that packs the punch in what you want it to do (blu-ray / hd rom drive will cost you a packet!) and get a decent external monitor / tv to plug it into. That way you can go much bigger than 20" for much less (dell's 24" is ~£550 if you know where to get it)
A friend of mine recently brought a dell inspiron 9400 and is loving it, though he got it at the right time (lots of freebie upgrades) keep an eye out for deals on it as he got a 2GHz core 2 duo, 120gb hard drive, 2 gigs of ram, 17" lcd and ati x1400 all for around the £900 mark
