How about this from Dabs. An Acer E500 at 450 pounds. Delivery may be extra.
Fast CPU, reasonable amount of memory, large hard disk, TV tuner card built in, flat panel monitor, DVD re-writer, Windows Media Center Edition.
http://www.dabs.com/ProductView.aspx?Quicklinx =43BW&CategorySelectedId=11101&NavigationKey=1 1101&InMerch=1
I know you said no jargon, but this PC has 512Mb of memory which is reasonable, To allow for future growth I would try to get double that at 1Gb.
Not the end of the world if you dont, but worth thinking about.
Apart from that, the machine above looks a good deal.
Note there are probably loads of other deals between 400 and 500 pounds so dont leap in with the first one you see.
See if you can get a 19" monitor if you can, rather than a 17" as you can see movies and things better.