I don't know how case fans bought locally fare against those bought from etailers in the UK. In the US, local retailers lose this argument easily. At least in my area. Most of them don't know what a fan is, much less sell them. One local retailer sells 80mm's however, they are wimpy--but then again, they're cheap.
Most cases are pre-punched with 80mm fan mounts. Unfortunately, two 80mm's(1 intake/1 exhaust) are no longer sufficient for overclocked boxen. If you're NOT running an overclocked T-Bird, two high powered 80mm's can do the trick. The catch is that you need to remove the blockage, in the form of the cutesy patterns of holes that cover fan mounts and interfere with good airflow. Unless you have money to spare for a brand new box with ALL the things that must go into it, you must remove everything from a case before cutting the blockage away. Then the case must be cleaned very carefully before putting the components back into it. One particle of steel dust can ruin your machine.