I'm not trying to say the GTX780 price is good, its anything but, however you are doing a bit of an apples/oranges thing there. The HD7970 you chose for the comparison is not a GHz edition and so the difference is >20%, more like 30%.
Obviously for the £330 you mentioned you can get GHz edition cards which are closer to the GTX780, but if your going to compare overclocked cards (the GHz edition being a factory overclocked 7970 released to combat the GTX680) then it makes sense to do so for both cards, and an overclocked GTX780 (such as the EVGA SC) will outperform the 7970GE and standard 7970 by 35% and 47% respectively.
*Performance figures taken from the TPU review of the EVGA GTX780 SC, combined performance chart
http://www.techpowerup.com/reviews/EVGA/GTX_780_SC_ACX_Cooler/26.html