What might help drop prices is that it seems that maybe a lot of AIB's are holding back cores for 5970 OC editions to release days after Nvidia announce their cards, that could see a decent flood of higher end cards around the world. IF that pushes normal 5970 prices back down towards RRP, which is quite possible/probable that will make the expensive 5870's drop in price.
Likewise it would seem a bunch of cards like MSI's better cooled cards, Gigabytes overclocked versions, MSI lightning versions are all coming out also to spoil the Fermi party. Again if say a MSI lightning or better cooled one is supposed to be £320, the normal prices have to come back down and maybe even drop towards £250-270.
However, £200 is fairly cheap for a 5850 card, they cost a lot to make and aren't making a humoungous profit on them. Theres not a whole load of space for the 5850 to drop in price with a crap 40nm process. £180 way in the future might be the best we can hope for, you're better off spending the extra £20 now and having it for far longer.
The MSI twin fan cooler 5850 is probably going to be the best buy around, great cooling £10 more, better/easier overclocking, will probably beat a 480gtx overclocked(as Fermi's are going to overclock like crap), for £200.
Frankly a £200 5850 or a £420(RRP) 5970 are the best value cards, everything else isn't worth getting.