CPU: Athlon 64 (Winchester core) 3000+ ---Get the retail box unless you find the OEM (meaning the CPU only) version for UK equivalent of $20USD cheaper or better.
Motherboard: Something with the nForce 4 Ultra chipset. Something like the Chaintech VNF4 Ultra runs right around the $100 mark here in the US. The DFI NF4 Ultra/SLI boards are also very good, but they might be a little tricky if you're not used to the sheer amount of settings. These are popular and from what I can tell, very solid (review of NF4 Ultra-D coming soon ;)).
RAM: 512MB minimum. PC3200 or faster speeds. Go with Patriot, GeiL or G.Skill for low-cost RAM that doesn't suck.
Video card: Look at the GeForce 6600GT in PCIe (PCI Express) format. This will work with the nForce 4 and should be easily had for under $200USD (probably under 150 pounds or so)
Hard drive: 120GB is about the sweet spot for price vs space.
Optical drive: Whatever seems to fit your needs, but I'd highly suggest at least a CD-RW drive for backups.
Power supply: Okay, this may not seem important, but it is. You want a 400-500W Power supply from Enermax, OCZ, Thermaltake, and possibly Antec. Do NOT get some $30 PSU sporting 600W. Thermaltake PSU's are pretty solid and relatively inexpensive. The other three are a bit pricier, but they're generally considered to be some of the best you can buy. I have been hearing some complaints about Antec's units as of late.
If you really have money to burn, PC Power & Cooling is excellent too, but these are very expensive (over $200 USD).
I'm not as familiar with UK sites, but feel free to run your system ideas past us. I've browsed Dabs and Komplett, but I know there are others as well.