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.
PCPartPicker part list:
http://pcpartpicker.com/uk/p/njHc
Price breakdown by merchant:
http://pcpartpicker.com/uk/p/njHc/by_merchant/
Benchmarks:
http://pcpartpicker.com/uk/p/njHc/benchmarks/
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)
Total: £567.92
(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.
Hope I helped!