Go with Antec or Seasonic for the PSU.
650w should be fine for a single card. Maybe 750w if you want to SLI/Crossfire.
Stock fans should be fine if you're not overclocking. Get the Hyper 212+ or NH-D14 if you are.
Go with 2x2gb of RAM. Also if you're on a budget, go with 500GB or less of HDD space; you can always add more later.
As far as cases go, just get one that fits all your parts with good airflow and cable management.
Also you're missing a PSU.
There's not much you can do with £300...
The Antec 300 is pretty much being phased out and replaced by the One Hundred, which has a few more features, weighs a bit less, and is a bit cheaper.
Also why wouldn't you want to overclock? The initial cost is like $10 more and will give you better performance in games than just throwing...
Depends on what you're looking for.
For general computing and schoolwork, I'd recommend a Thinkpad. Expensive, but top notch build quality. Otherwise something from Dell or HP would work, but keep in mind these laptops generally use Intel IGPs which are generally terrible for gaming.