I can get the base 13" on the FE discount for £729ish I believe. People who qualify for HE (university) get a bit of a larger discount, roughly 3-4% more. Students are allowed to purchase 1 x Notebook and 1 x Desktop on the discount each ACADEMIC year. So unless someone has bought a new one within the last week, just find a student (like me) who will buy you one, so long as you give them some kind of profit.
Dixons online wants £819.90, so they do give a little bit of a discount (£80) on it. And as said, if you find a Dixons Tax Free in an airport (I am always using the one in Manchester Airport) I'm actually buying my 5th Gen iPod Nano in there when I'm next in there in 3 weeks (y).
But to be honest... £700... You're looking at a used late2008 one to be honest..
Good luck.
EDIT: As I said, I just had a quick look on eBay, there's 2.0Ghz models of the Late 2008 MB on there, for £600-£700. Best I can find. Sorry!