Just had a quick scroll through the first config you posted. If that comes out at £274, i really think it'd be worth raing up a little bit across the board. For example: A mid tower or a large tower will REALLY help with cooling ALOT and give you a bit more longevity in the system. The Thermaltake Element t-mid is a substantially better case for a little under £20 more. Again, for another £10 you can get case fans, etc etc. I know the costs mount when you upgrade everywhere 'just a little bit'- I've done it.
Advice other than that is make sure you're getting reputable brands renowned for making the components. PSU especially, make sure that's good quality - if this fries it's hardest and most time consuming to replace (after motherboard) and if your rig 'just won't turn on' it can be a real pain identifying the issue, so if you have a good quality supply you can be a little more confident and swift in remedying the issue.
Also, OS wise, if you're not getting one bundled you can pick up an OEM copy from the net pretty cheap. make sure you go for genuine win7 home premium. You won't miss any of the pro stuff, I assure you. Also, 64bit OS, or you're crippling your system straight out the box.