Tough choice, isn't it? The MSI GeForce GTX 770 Lightning Edition is an amazing card, and there's nothing out there that really competes with it, but is cheaper. Palit's GeForce GTX 770 JetStream 4GB GPU is quite nice, with double the RAM, something else you should consider.
The PNY GTX 770 Overclocked 2GB GPU is £48 cheaper, but won't overclock anywhere near the levels that the MSI Lightning GPU will. If you want something cheaper, there's a ~15% saving right there.
As for matching the GTX 780, an overclocked GTX 770 will get you close. I would recommend the MSI GTX 770 Lightning if you don't mind spending the extra cash, but the Palit JetStream is another huge contender for your cash, too. If you want to spend the least amount of money, the PNY GTX 770 Overclocked 2GB is there for the picking.