Whilst I am sure you can undercut Cat's selection at a computer fair, I am also sure it will be at the cost of quality.
If you want to bring the price down, how about:
http://www.ebuyer.com/product/237554 £42.94 Dual core X2 3.0GHz
http://www.ebuyer.com/product/190138 £58.89 HD 5670 512MB GDDR5
Today's prices that brings the system down by about £40. It is a slower machine, but faster than the one my wife is still happy with
I'm sure you could get a cheaper motherboard as well, but that would be at the expense of limiting CPU choice to 95W, only USB2 etc and if you want to upgrade in the future it is a pain. Most people never upgrade a machine once built though, so perhaps this is an option.
I wouldn't want a cheaper hard drive, you would lose too much performance.
How much cost do you need to lose from Cat's suggestion to get to a happy place?