Currys and PC World both do refurb laptops and have some for under £300. Tesco has a half decent Acer with the nice 15.6" screen for £340 and I`ve got 10% discount, so would work out just over £300. Basically if You're from Edinburgh You can borrow the card as sometimes they do clearance sales of stock that is no longer in stock on their website.
Or just get something half decent off ebay for around £150 also Gumtree is worth checking. But if I would be getting a used laptop for £200-250 I would rather pay extra and have a new one with warranty for £300.
Dell has an internet outlet and You should be able to get a refurb inspiron for under £300.