Well,
****** . It seems that almost all (if not all) 7850's will have aftermarket coolers upon release, and the one that I think you are taliking will not be strong if their previous budget end (non-DirectCU) coolers are anything to go by.
Out of the 570 and 7870 I would go for the 7870 if I really had to chose, mainly because I have already owned most of the GTX4xx, GTX5xx and 69xx series cards. I llike new and shiny

.
The 7850 @ £185 is a better bet than a 7870 @ £270. It may have 20% fewer shaders, but once clocked to 7870 speeds it is only about 8-10% slower. ~90% of 7870 for 65% of the price seems pretty good to me (and yes, I know the 7870's overclock better).
The 7870 is overpriced compared to current alternatives (including the 7850, 6950, GTX570), and also baring in mind it's low production cost. The 7850 is okay'ish, being the best value 7x00 card so far, but still does not win any great value medals.