£350 for a 770? sounds to good to be true. I don't see the 680 prices budging, and with the 780 at £550 I fully expect the 770 to be £400-450.
If it is £350 then wow... I might even return my 780 for a 770.
http://forums.overclockers.co.uk/showthread.php?p=24325387#post24325387

"770 is 680 price level
Bear in mind were selling 680's now close to £350 as 770 will eventually replace that model. "
Sounds to me like it will be £350, maybe even less.