A 285 for over £230, when a 5850 demolishes a 285GTX and can overclock by a pretty damn easy 25%, and up to 35%, which is well beyond 480gtx performance, for less than a 285GTX which is also "on offer".
Honestly, if you really won't go AMD, get the cheapest 260gtx you can find, 2nd hand maybe, new 260-285gtx's are horribly value, they were cheaper 6 months ago and have only aged further in terms of relative performance to other cards around. Heck a 285GTX is worse value than a 480GTX if you ask me.
Go second hand or, well you'd have to say what you have to be honest, if you're way way back on a DX9 card, a gts250 wouldn't be too bad. It really depends completely on what you want it for, high end gaming and you'd really have to be mad to not get a 5850. Something for occasional gaming or low res, a 260gtx second hand probably would end up a great card.
As for the future of Nvidia, their derivitives will be power hungry, large, expensive and late also, honestly I wouldn't be counting on anything in the 4xx series being great. A 2xx card if you can find one for good value, or second hand, is your best bet by a long long way.
Keep in mind that in September last year £130 260gtx's were pretty common place, and 285GTX's were routinely being sold under £200, the artificially high prices how are to discourage people buying the last few. I'd wager, especially now and as the 470gtx hit the shelves, that 260gtx's will be pretty widely available second hand, as some people get 470gtx's but a lot decide to go with 5850/5870's aswell.