www.ebuyer.com
Or, just a thought, if you are a registered charity, you are entitled to a special discounted price from the major IT resellers (ie ebuyer, dabs, pcwb). I know this is the case for Hewlett Packard and they have a low spec basic laptop (53 series) that may be close to your price range with the special bid. Not sure about the other manufacturers
PM me, and I'll let you know what you need to ask for