£230 isn't high end. £450+ is.
4850 are slated as $199 and the 4870 are $349. Current exchange rates (now not 2$ to the pound), VAT and duty get you to around £230 for the 4870. Widely reported but we will know for sure in two days.
ATI recently reported to have upped price of 4870 by $50 recently so it should have been just under £200 over here originally. Not twice the price even at £230 but would have only been 50% more money originally.
However, you have to look at the cards they are competing against. You just can't say that ATI won't be stupid enough to charge almost twice the price for 30% gain. Nvidia is doing the same for 18% gain over a GTX260 (although slightly different since the GTX280 is the fastest card you can buy from anybody).
The 4850 is to compete with the GT. 4850 expected to be 40% quicker so hence £130 is a good price. 40% quicker but only 20% more money than the competition.
4870 competes with the 9800GTX at £220. £230 for a 4870 which is supposed to be 40% quicker is a steal as well. Why sell the 4870 for £20 less than the competition when you are beating them by 40%? They will sell by the bucket load at £230 if they deliver the performance. How much more you charge for your other cards become irrelevant.
Taking the 9800GTX example from Nvidia, it costs twice as much as a GT but is only about 25% quicker.
Graphics cards don't scale on price/performance.
And finally, the rumours about the price increase by ATI seem to stem from the performance being better than expected (of both cards). If the 4870 is matching/beating a GTX260 then £230 seems a fair price.
Hopefully, since the 4870x2 isn't out for a couple of months, by then both ATI will have dropped their prices but without doubt, going on ATI's previous pricing, then 4870x2 will be twice the price of whatever the 4870 is when it is released. Even if the 4870 is £180 as you state, it will still make the 4870x2 £360.
Hell, if the 4870 comes out at £169 (30% more than the 4850) and one 4870 does beat the 9800GTX by 40% and hence matches the GTX260 for £270 I will be buying two of them on launch day within seconds.
Two 4870's for £338 would beat a GTX280. SOmehow, I just can;t see ATI selling their cards that cheaply if they have the performance.
The only way they will be that cheap is if the 4870 only matches the 9800GTX in performance IMO in which case I won't even be buying one.