I'll assume that the parts can be had in the UK for a similar price as the US and that 1 GBP = 1.75 USD. Ergo, I'll work with a budget of $1,750 USD.
Altlon X2 4400+ (or Opty 175 which is equalivent)
2 MB Cache (1 MB+1MB)
THG CPU tests show that video editing makes great use of dual core processors. You can go lower end dual core but the 4400+ and the Opty 175 are easily OC'ed to achieve 4800+ speeds without any effort. I suggest the 4400+ unless you are a serious OC'er because it is less expensive ($464 v. $513).
X1900 - while I am a nVidia guy, I can't contest that the X1900 is the best price/performance graphics card out there these days for the high end. Costs about $580.
RAM - 2 GB OCZ Plat. $220. Some of the best memory out there today (no disrespect to Cosair)
Currently at $1264 for the above parts.
2 Lite-on DVD recordable drives. If doing video editing, you'll want two DVD drives. ~100
MB - Couple of options. You can go with the Asus A8R MVP or the Abit AT8 ATX - both are nice boards from good mfgs. I'd suggest DFI, but their MB uses fans, instead of passive cooling, which I have been gravitating away from. ~120
That's $1484
Seagate SATA 250 GB HD - Great dives. ~100. Raid would be nice but I think a second would put you over budget.
That's $1584 which leaves ~200 for a case and PSU.
I am not including keyboard, mouse, moniter as a lot of people are just ask about the base computer. Obviously, if you don't have these, the above budget is useless.
Optional - soundcard. Prices vary widely.
This system would handle any video editing you threw at it with style.