Depends on what you're after. Any DRZ for that money will either be knackered or an absolute bargain so you might get lucky. Beyond that you'll easily find an older bike for that money, it just depends what cc you're looking at.
I'm in the same position as Jimbo. I've looked at ex-army MT 350's, but they seem to go for around £1500, which is just over my budget. People sell unregistered ones for £1200, then there's a cost and hassle of getting it registered, taxed, MOT'd etc etc.
Perhaps me and Jimbo are being unrealistic in getting a deasonably decent green laner for a grand?
Like D6 says those are decent models to look for with a smaller capacity. I've also spotted a TTR250 for £700 on Pistonheads (up in Cumbria so too far away for me I think). EDIT: And looking at both of your locations you're probably no more than 30 mins from me (15 mins in the case of Slowdown!) so it's probably too far for you too.
If you want something bigger you might have to look at something a little older, or search for a project if you don't mind the work.
I just flogged a DR350 for 800 pounds and they do OK. Watch Terra Circa and Mondo Enduro. They didn't keep snapping frames and popping rear shocks like those shit BM;s Ewan and Charlie struggled on