I would think these would be in the region of £189.99
By todays trumped up "haha you're uk consumers and therefore suckers" prices, maybe, its rrp will probably be around £150, but remember at some point the 4890 won't be being made anymore.
The choice isn't crystal clear which one you should buy right now, in 6 months you'll only be able to buy a 5830 though. They seem to be waiting on TSMC to switch more production from 55 to 40nm at the moment though, as ideally you'd normally get 40nm way cheaper comparitively than 55nm and TSMC seeing the value and all their customers wanting it would switch most of their normal production over. However with 40nm sucking balls, the price improvement over the previous process is simply not existant, so the push by customers to spend a lot of money remaking chips just for a lower process isn't there, so TSMC's push to very expensively(in the billions) switch more and more production has also been non existant.
AS for competition, there is no competition, the 4870 brought the GTX prices down because they were vastly, VASTLY cheaper with similar performance, if the 4870 came out at £250 and the GTX280 had launched at £350, why on earth would that have forced 4870 prices down, it doesn't work the other way around. A more expensive Fermi, with non existant high end parts at stupid prices won't push prices down.
Yield improvement would, but thats not going to happen, you're not going to go from 40% yields, to 80% yields in the space of a couple months on a year old process with nothing new to improve it. The 58XX's cost an extra $100 or so as they leave the board makers for distributors, not in $100 more profit, but with $100 higher production costs. A 5850 will never cost £100-125 like the 4850 did, if it had twice the yields, it would.
My guess would be though that the 5830 shouldn't have any issue overclocking like the 5850 can, and depending on how many missing shaders compared to a 5850, it could quite easily spank a 4890. If its a 1000-1200 shader part, with dx11, can clock to 5850 overclocking levels with a nice new unlocked bios and has increased rops over the 4890, it will be a killer card.