Just to add a further option: http://www.ebuyer.com/product/140194
Fujitsu-Siemens Primergy Econel 100, PD 925, 1G, 160GB HDD, DVD-RW DL\DVD-RAM at £110+VAT = £130, delivered.
I'm biased towards the Fujitsu-Siemens as its a brand we resell, although I couldn't beat the above price. We've used several of these beasts for Linux jobs, although as servers (ie I've not really cared about the graphics, they've all been GUI-less, so I can't comment on that - I'm sure we've had some running Ubuntu with a GUI in the past without problems though).
If the GUI matters, they also now have a Linux desktop: http://www.ebuyer.com/product/143780
Fujitsu-Siemens Esprimo Edition P2411 Sempron 3800+ 1GB DDR2-667 CD-RW \ DVD SATA = £120+VAT = £140 delivered
We've used a number of these in their Windows XP flavour for customers without problem (and usually at a good deal higher price than that, so its nice to see the lack of MS tax). I've not tried one of these yet as a Linux box - only discovered it yesterday - and have no idea what flavour of Linux is on it. Its very tempting though...