Everything looks fairly well balanced, although if you're not overclocking, you won't need a Z77 motherboard. You could save a bit of money on that, but otherwise, everything looks fairy balanced.
It seems solid for a balance between gaming and productivity, however if you wanted to purely game...
If you're going pure gaming, you could do a little bit better. 8GB RAM would be standard for a gamer's build, and you wouldn't need 2TB of storage for gaming.
I wouldn't change much, but here's how I'd build it. I'd stick with a lower clocked i5, since clock rates don't affect gaming as much as higher end video cards, get myself a B75 motherboard since I wouldn't be overclocking (B75 is like a Z77 with some trivial features missing, like overclocking), downgrade to a 1TB hard drive, get 8GB of RAM (most modern games hardly use 4GB in the first place), and use the saved money to purchase a much, much better graphics card.
This build is better for gaming, and you'll save a few pounds.
CPU: Intel Core i5-3350P 3.1GHz Quad-Core Processor (£132.85 @ Scan.co.uk)
Motherboard: MSI B75MA-E33 Micro ATX LGA1155 Motherboard (£45.30 @ Scan.co.uk)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£30.96 @ Scan.co.uk)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£52.88 @ CCL Computers)
Video Card: XFX Radeon HD 7950 3GB Video Card (£216.98 @ Dabs)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply (£49.98 @ Novatech)
Optical Drive: LG GH24NS90 DVD/CD Writer (£13.98 @ Novatech)
Other: Casecom KM-6988 Black Mid Tower Case with Full Black (£24.99)
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2012-11-11 19:02 GMT+0000)
If you wanted to keep that CPU, and also if you really want 16GB of RAM, this build still fits under 600 pounds. The changes I made weren't huge changes, except for the graphics card. You could get possible 20 or more fps in games with a 7950 vs. a 7850.
The Mobo will fit the case just fine. Also, as far as gaming is concerned, your processor won't have any problems. The graphics card is far more important. For example, an i3 with a 7950 will blow away an i5 with a 7850. Your processor is great and will be for some time.