OK, since you stated that you had your monitors and software already taken care of, weren't planning to worry about acoustic treatments at this time, and (I'm assuming) already have decent microphones, then all you should need is a PC and an interface with built-in mic pres to get going. One problem I had is that I'm not from the UK, and don't know the best online sources there, and as I investigated I found that the prices seemed to be somewhat higher than in the US. However, I did find the following interface for you at an online UK music source:
http://www.dolphinmusic.co.uk/page/shop/flypage/product_id/7768
This is a firewire interface from a manufacturer with an excellent reputation. It will allow you to record stereo tracks through the two mic pres, as well as through the line inputs. It also has outputs for hooking up your monitors.
After purchasing the interface, that left £450 for a PC. I searched around and found an online UK company with a "configurator" which lets you specify the options you want and gives you a quote. Here's a link:
http://www.more-solutions.co.uk/shop/custompc.html
Using this, I spec-ed out the best system I could do for a recording worthy PC keeping the budget at the £450 range. This is what I came up with:
Processor AMD Sempron 2400 £183.75
Case Full ATX
Memory 512Mb DDR £16.25
Hard Drive 40GB 7200RPM
2nd Hard Drive 80GB 7200RPM £40.00
Optical Drive CDRW/DVD Combo £22.50
Graphics Onboard
Operating System XP Home £61.25
Monitor 17" CRT £61.25
Warranties Standard 3yr RTB
LAN Onboard 10/100
Keyboard/Mouse Windows Keyboard & Optical Mouse £3.75
Total
VAT £388.75
£68.03
Total (inc VAT) £456.78
As you can see, we've now exceeded your budget by about 6 pounds. I would've liked to do a little better with the PC specs, but this will work. If you can come up with a little more dough, however, I would suggest a few upgrades to this spec as follows:
1. Go with one of the better AMD processors, such as the Athlon 64 3400+.
2. Increase the RAM to at least 1 GB, or even better yet, 2GB.
3. Increase the size of your second hard drive for higher audio file storage capacity (Just make sure whatever you get runs at 7200RPM).
4. Replace the 17" CRT with a 17" LCD or better yet, a 19" LCD monitor.
One other thing to keep in mind is that you need to make sure your PC has a firewire adapter for hooking up your interface. Many PCs come with them, but the source I was using didn't state one way or the other. So before ordering I would contact the company and ask. If it doesn't come with one, ask how much it would be to include one. You can usually get a firewire adapter that plugs into one of the PCI slots on the PCs motherboard for minimal cost.
Hope this helps - let me know if you have any questions.
-Jeff