Well it was worth £600 when Ghz 7970's were £350+, not now they are available for £250 which means 2x 7970Ghz = £500, would make a 7990 expensive at £600.
What your forgetting is 7970 is EOL. So when the deals are gone that is it, you've missed out. 7990 is current and won't be EOL until 2014.
670 and 680 deals proof of that, the crazy pricing is long gone on those and won't return as NVIDIA have now cleared the stock and our stock is minimal so we don't need to drop to such low levels again.