If you're only going to be using the computer for the internet, �300 is far too much to even consider paying.
If I was in your position, I'd simply buy a refurbished base unit from somewhere like A C Computer Warehouse. (I can thoroughly recommend them). Their prices (including the VAT which isn't shown on their index page), start at �35 for a Celeron 900 with Windows 2000, passing through the �110 mark for a Celeron 2.0GHz with XP, up to �180 for a HP Pentium 4 with XP.
Even the �35 unit is perfectly good enough for internet browsing. It only has a CD-Rom drive but, for an extra �34, you can upgrade this to a DVD ReWriter.
http://www.accomputerwarehouse.com/products/Re furbished-Base-Units
Chris