As mentioned above you are going to get a bit of flack from some people for not building your own machine. All I can say is this forum will help you choose the right components and any issues run into, but you will have to do the work and feel comfortable with the process. It will take time and patience. If you just want a nice gaming rig and not worry about the PC hobbyist part of gaming, then that is fine to.
As far as your configuration goes it looks fine to me, I can only offer my suggestions. See below.
1) Since you are in the UK I would research what companies make the best gaming PC's for a decent cost. I am in the US and there are some definite go to sites for this sort of thing, CyberPowerPC and iBuyPower. You just want to make sure to get at least a somewhat reputable company in case there are issues with your PC while under warranty. Some small shops may have slow turn around and limited support. Just do your research first, don't just choose the cheapest option.
2)Make sure the motherboard you choose has the features you would like to have. I see you chose a B75 chipset, which should be fine. The only things you will want to make sure of is it has the features you want or think you may want down the road.
3)The only component item I could suggest changing is the video card. I would change the GTX570 to the GTX 660 or potentially the 660Ti. For a gaming rig you want all the video you can get.