At that budget I would suggest going AMD as it is cheaper and the socket is more upgradeable (you don't have to replace the motherboard to upgrade the processor)
Gigabyte GA-MA770T-ES3 - £58.71
http://www.scan.co.uk/Products/Gig [...] s-RAID-ATX
AMD3 440 3ghz Tri core - £61.62
http://www.scan.co.uk/Products/AMD [...] 95W-Retail
Samsung Spinpoint F3 500gb - £34.89
http://www.scan.co.uk/Products/500 [...] he-8ms-OEM
Dual Layer DVD Drive - £13.88
http://www.scan.co.uk/Products/Sam [...] -Black-OEM
CASE - Xigmatek ASGARD - £26.78
http://www.scan.co.uk/Products/Xig [...] Fan-No-PSU
Silverstone 500W PSU - £43.39
http://www.scan.co.uk/Products/500 [...] -120mm-Fan
G Skill Ripjaw RAM - £97.99
http://www.ebuyer.com/product/180490
Total: £337.86
For monitors, either 24 inch Acer - £158.60
http://www.scan.co.uk/Products/24-Acer-V243GBD-Black-Widescreen-LCD-Monitor-1920x1080-400001-DCR-300-cd-m-2-ms-VESA
or the LG 23 inch - £159.55
http://www.scan.co.uk/Products/23-LG-W2361V-Black-Widescreen-LCD-1920x1080-500001-DCR-300cd-m-2-ms-Speakers-1080p-Full-HD
Total: ~£500
In my house I have an LG monitor and my wife has the Acer quoted above. It uis up to you which you go with, the LG is a bit smaller for the same kind of price but is technically a better brand quality - although ironically, at home my LG monitor has got a dead spot on it right in the middle of the screen now while the Acre monitor is still perfectly fine.
I just saw that you are not including the monitor in your budget, that is good news, so scrap the CPU and motherboard I mentioned as you can get better. So, without monitor:
i5 750 - £158.60
http://www.scan.co.uk/Products/Intel-Core-i5-750-Lynnfield-45nm-Quad-Core-266GHz-8MB-Cache-95W-Retail
Gigabyte GA P55-US3L - £77.80
http://www.scan.co.uk/Products/Gigabyte-GA-P55-US3L-rev-2-Intel-H55-Express-Sok1156-PCI-E-20-DDR3-2000-SATA-3Gb-s-RAID-ATX
Total: £453.33
This build does not allow for XFire or USB3.0 and Sata 6gb/s, but allows spare cash for a CPU cooler for OCing and posibly a more powerful Powersupply.
The best P55 board that will give you USB 3.0 and Sata 6gb/s and XFire is:
Asus P7P55D - E Pro - £163.97
This will break your budget at: £539.50
If Crossfire matters to you but USB3.0 and Sata 6gb/s does not, you can likely buy a board in between. (For me personally the USB3 stuff was of no interest, it is something that will matter when I get a new system in 3+ years).
Depending on games you play, you may find the 5770 starts to struggle at max settings on some games, but if you have an XFire board you can always put a second one in, or if not buy a 5850 in the future, but other than that the computer is good to go!
Note that I bought the RAM from www.ebuyer.com this is because, while I love scan (I have used them for years) the one thing they lack is RAM variety - G Skill is brilliant RAM and tends to have lower latency than the Corsair on offer at scan, so I always buy the RAM elsewhere.