What is the best £450 gaming pc build?

Well here's the best I can do:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-6300 3.5GHz 6-Core Processor (£83.99 @ Aria PC)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard (£59.00 @ Amazon UK)
Memory: GeIL Enhance CORSA 8GB (2 x 4GB) DDR3-1600 Memory (£40.99 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£42.00 @ Aria PC)
Video Card: XFX Radeon HD 7870 XT 2GB Video Card (£133.99 @ Aria PC)
Case: Corsair 200R ATX Mid Tower Case (£47.98 @ Amazon UK)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply (£49.24 @ CCL Computers)
Optical Drive: Lite-On iHDS118-04 DVD/CD Drive (£10.78 @ Scan.co.uk)
Total: £467.97
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-11-17 19:58 GMT+0000)

Great processor for the price with a great quality motherboard. Add a cpu cooler in the future and overclock it.
7870 XT can play games on med-high, depending what you play. XFX isn't the best brand, I actually never recommend XFX, but a 7870 XT at this price is amazing.
Great quality case and psu for future upgrades.

Don't expect this to play games in 4 years without a single upgrade.