Quality and aircooling its the CM 840 with your budget, the Corsair isnt that great imho.
Theres the lian li P80, and the B70 aswell, really depends what you are going to do with it dude? I will say with a £300 case budget if you are into gaming Id buy a better GPU though.
If you want DX11 then Id just swap the budgets around, spend 300 on the gpu and 150 on the case. This means buying something like a Cooler Master HAF 392 and a 5850 or a 5870. Theres plenty of cases come in at that price point but imho the HAF is very good value for money, and as a build it and leave it case you cant go wrong.