A good place to buy cheap/2nd hand laptops?

[NO-DRAMA 10]

Hi can anyone recommend a good place to buy used laptops?Or cheap ones?I need two,one for each of my kids for Christmas.My budget is £150 for each one and need to be either XP or Windows7 and capable of streaming videos.I live in S8 in the south of Sheffield and I don't drive so anywhere round there or city center would be great.thanks:)