Another option is:
http://www.arbico.co.uk/Arbico-Core-i5-6870-Pro-Custom-Built-Gaming-Computer-p-.html
Specs:
CIT Jupiter Black
• 2 x 120mm Standard Case Fans
• Quiet Corsair 600W
• Intel Core i5 2400, Quad Core (1155, 3.10GHz, 6MB Cache)
• Standard Heatsink & CPU Fan
• Standard Heatsink Paste
• Asus P8H61-M X, 2xDIMM, Max. 16GB, M-ATX
• 8GB (2 x 4GB) Kingston 1333MHz DDR3
• Overclocked ATI Radeon HD 6870 1GB
• 500GB SATA II
• NO RAID
• 22x Dual Layer DVD +/- Rewriter
• Integrated 10/100/1000Mbps
• 2 Front + 4 Back Ports
• Integrated 7.1 High Definition Channel Audio
• Operating System Genuine Windows 7 Home Premium (64bit Version)
Total (including delivery) = £652.99
The advantages of this setup are a better case, it looks better and probably has better cooling (it has 2x 12cm fans). I upgraded the PSU to a Corsair (+£40) for peace of mind and it comes with a more powerful GPU (AMD 6870). If you go for this make sure that you select the 6870, as the default is set to Nvidia 560 (non ti) which is considerably slower. The i5-2400 is slightly worse than the i5-3450 but it's still faster for gaming than anything AMD has to offer.
On trustpilot.com arbico.co.uk scores an average of 9.1/10 with 58 reviews, so not bad but not as confidence inspiring as the stats for pcspecialist.
The reason I upgraded the arbico PSU to a corsair is because the default was a 500w coolermaster which I strongly suspect is a load of rubbish. The 450watt PSU offered by pcspecialist I figured was probably better because it is a 80% efficient one.
PS. if you click that link you will see the price is listed as £589(inc VAT). With delivery and the upgraded PSU it comes out at £652.99.