Crossfire is having the bugs ironed out so I agree with the other post.
Also it might be worth looking into a Corsair, XFX or Seasonic PSU - nothing wrong with yours but there are better ones around for similar prices.
And finally you will save £100-£150 if you build it yourself. It really isn't difficult and you could spend that saved money on better parts.
The PSU Choice is limited to One PSU per each Wattage. Also I have heard that AMD has many problems with Crossfire and that Nvidia is more reliable with it's SLI.
About the GTX 770 4GB, it is 45 pounds more expensive and I am not sure if it is really worth it...
That said, would you say: GTX 760 4GB or Radeon R9 280X 3GB?