I am an Antec fan and have used the 300 series in probably 25 builds. I have never used the Bitfenix but it also has good reviews. Go with the one you like best. Both are very good but you are the one that has to look at it everyday!
At 1366 x 768 that is a good build. However you might want to look at a slightly less powerful CPU and a better graphics card. A well balanced gaming computer should have a video card twice as expensive as the CPU. So maybe look at an i5 3330 or i5 3450 and a GTX 760 or even a GTX 770 if you can fit that in your budget.
I would be fine using that power supply on a system with a GTX 760 or less in it. Anything more powerful and I would step up in quality. In fact for 8 more pounds this is a better unit although not modular. It would be fine for a GTX 770.
http://www.amazon.co.uk/XFX-P1-550S-XXB9-PRO550W-Edition-Supply/dp/B004RJ8EKI/ref=sr_1_1?s=computers&ie=UTF8&qid=1375413760&sr=1-1&keywords=xfx+550
Well as for the case I think I'll choose Bitfenix as I love the looks, it's features and slightly lower price. As far as the balanced build, man are you crazy !? That i5 is just ten pounds cheaper, and the graphics card will make it 750£ + ! That's like 900 euros, there's absolutely no way I have money for that, 600 is already pushing it. The only option would be i3 3220 + gtx 660, but I'm afraid that means bottleneck, although it would most likely have superior performance due to the much powerful gpu, but I want a quad core to play games battlefield 3 multiplayer, gta iv, and also for upcoming games that will probably benefit a lot from a quad core.
I do play games a lot but I don't mind reducing to 720p and for that resolution I don't think gtx 650 ti will have any trouble honestly, and even if it does somehow I just need to turn the settings down a bit right?