Intel Core i5 2500K 3.3GHz Socket 1155 6MB Cache OEM Processor
http://www.ebuyer.com/product/244099
Kingston 4GB DDR3 1333MHz Memory Module Non-ECC CL9 1.5V (2 of these for 8GB total)
http://www.ebuyer.com/product/179755
Gigabyte GA-Z68P-DS3 Z68 Socket 1155 8 Channel Audio ATX Motherboard
http://www.ebuyer.com/product/284388
Sapphire HD 7870 GHz Edition 2GB GDDR5 HDMI DVI Dual Mini DisplayPort PCI-E Graphics
http://www.ebuyer.com/product/349749
Antec 300 Three Hundred Two Case - with USB3.0 on front panel
http://www.ebuyer.com/product/341398
Arctic Power Pro 850W Modular 14cm Fan PSU - 4x PCI-E 8x SATA
http://www.ebuyer.com/product/229935
Arctic Cooling Freezer 13 Intel and AMD Socket Heatpipe Cooling Fan
http://www.ebuyer.com/product/253477
Kingston 120GB V+200 SSD - 2.5" SATA-III - Read 535MB/s Write 480MB/s
http://www.ebuyer.com/product/341349
Total Cost: £798.18 (with free delivery option)
Opinion: Having a beefier PSU will help if you choose to X-fire later. The SSD will make you productivity use much faster and pleasant as will the far superior CPU.
Having USB 3 front connectors is surely a must for a new build these days. Hence case selection. If you need more storage I suggest pulling out your current HD or finding a cheap deal at a later point. HD's are still overpriced from the flooding to the point I recommend buying second hand or waiting until they drop again, SSD's just offer too much in comparison.
GPU wise I would say the performance between the 7870 and gtx580 is much of a muchness, but power draw and temperature of the 7870 is far superior. Also possibility of multi monitor gaming is a big draw as it is now realistic on a single GPU like this.