I recommend
this thread - a lot of discussion of good cheap basses in the UK. Unfortunately, 'cheap' for a bass is still pretty expensive by any other standards (particularly in the UK, where the market seems much worse than the US), and £700 seems to be skirting the lower end of what people seem to think you're likely to be able to get a reasonable bass for.
You might be able to get a Stentor Conservatoire in your price range, which Ollie Brice has been saying good things about. You woud probably have to spend some more getting a luthier to set it up properly, though - don't assume that a bass, particularly mail order, is necessarily going to arrive in playable condition! Ones from specialist bass shops should do, though; or you can get a mail-order bass then find a luthier to give it some attention (but be prepared for this to cost another hundred or so - it is, however, well worth it if it turns an unplayable bass into a nice one).
The alternative is to be patient and looking out and ask around for second-hand basses - in that price range you might very well find more interesting things in second-hand than new, and a second-hand bass (from a reputable seller, at least) is likely to be in playable condition; whereas a new bass will often need attention from a luthier. The Violin Shop in Glasgow had a second-hand German laminate bass in for £750 last I checked - it may well have gone, but worth a look. (You don't say where in the country you are - I recommend filling in your profile.) I had narrow misses on a couple of old laminate basses around the £600 mark before finally getting my 1920s carved bass for (at the very limit of my budget) £1100. Of course, with second-hand basses you have to be sure of the condition of what you're getting; either buy from a specialist with a good reputation (ask on here if you don't know) or find an experienced bassist to take with you and cast a more practiced eye over whatever you're considering. There should in many ways be
less risk with second-hand, though - if it's survived for 10, 20, 50 years without obvious issues it's most likely OK for another 50; whereas a new bass just out of the box might be a time bomb that will collapse in a month.
Oh, and because I haven't got anyone else to bite yet -
this place seems to have a Boosey and Hawkes bass for £295, which is the cheapest I've seen a bass in
any condition. Caveat emptor, but it
might be a bargain - I'd recommend taking a knowledgable bassist along to see whether it is or not, though :-)