As other's have said, this site has great guides on how to build a PC - They are worth a read. Making your own PC is the best way to do it, and usually cheaper. Over here, best places for everything PC parts wise is
http://www.overclockers.co.uk or
http://www.scan.co.uk - They are constantly at war with each other over prices, so there's usually never anything in it!
Assuming your gaming at say 1080P, you won't need to fork out a lot for something that will do the job:
Processor: Intel Core i3-4150 3.50GHz (£89.99)
Motherboard: Gigabyte Z97P-D3 Intel Z97 (£69.95)
Memory: TeamGroup Vulcan ORANGE 8GB (2x4GB) DDR3 PC3-19200C11 2400MHz (£55.99)
Graphics: EVGA GeForce GTX 750Ti SC 2048MB or Sapphire Radeon R7 265 Dual-X 2048MB (£109.99 - £110.99)
Hard Drive: Seagate Barracuda 7200RPM 1TB SATA 6Gb/s 64MB Cache (£43.99)
Case / PSU: OcUK MJ-705B Micro ATX with USB3.0 and 500w PSU (£32.99)
Comes to £417 including taxes and delivery. You can spend a bit more moving up to an i5 CPU (£50 approx.) or getting a better case and separate Power supply (PSU) which I would recommend if budget allows.
Also, don't forget to add the cost of Windows on top of that if you don't have it. That's an extra £69.99 for Windows 8.1 64-Bit OEM.
Also, if you ask Overclockers.co.uk really nicely, they will build one for you. Fully custom, and a fairly reasonable price:
http://www.overclockers.co.uk/productli ... catid=2473
Mega Beast - Intel i7 4790K | Gigabyte Z97X-UD3H-BK | 32GB DDR3 | SLI GTX 1070 | Samsung 850 Evo 1TB
Mini Beast - Intel C2E QX9770 | Gigabyte X48T-DQ6 | 16GB DDR3 | KFA2 LP GTX 750Ti | Seagate 2TB SSHD