For the price, it isn't great.
You don't need 2 DVD burners, you don't need 2 hard drives, you don't need 2 5450's, you don't need a case that expensive.
I've just put the exact same system together from parts sourced online for much less than you have had it priced at there with the exact same specs, however you can spend the same amount to get much better performance, or even spend less to get slightly better performance
Honestly, if you are getting it prebuilt because you don't know how to build computers, or don't feel confident building them, you aren't going to get much more for your money. You will be able to get more, but not a whole lot, because alot of these places charge a fair amount for labour (in this case, pushing £100)
As you can see from the left, I'm from UK too and build/maintain/repair systems in my local area as part time work for extra money, however have done a couple for those further away (built a couple of systems for some people near london, a few in Somerset etc) so have sets of systems together if anyone gives me a budget and a requirement at any time. Have a £750 system (inc. VAT, NOT inc. shipping), fully built/tested before being sent out with following specs:
Motherboard: MSI P55A-G55 (Intel P55 1156)
CPU: Intel Core i5 760 Quad Core Processor (2.8GHz 8MB Cache)
Memory: 4GB 1600MHz Corsair Dominator (2x2GB 9-9-9-24)
Video Card: EVGA GTX 460 1GB
Hard Drive: 1TB Samsung F3
PSU: 550W Corsair VX550W
Optical Drive: Samsung ±DVDR
CPU Cooling: CoolerMaster V6GT
Case: Cooler Master CM Elite 690II
Can include Win 7 Home Premium x64 for an extra £80
That is a better build than the one you just put up as it is more balanced, more powerful, and much better for everything you do from everyday tasks to gaming, and the reason for that is the companies like YoYoTech charge massive amounts for not very good parts, and massive amounts for labour like I said before, where as myself, and if there are any like myself in your local area, tend to charge less for labour, and you get a more personal approach with them than online businesses, or large companies. Also building locally will mean no shipping costs
Honestly to get the best system for your money, you have 2 options:
Build it yourself if you can
Get someone locally/someone that builds systems that isn't an online store to build it for you