Minimum qualifications are "ability to walk upright" and "possess opposable thumbs".
That's a bit off. I've worked with some fantastic ATSAs. And a good one is worth their weight in gold. I'm sure many ATCOs will agree.
MetalThrower, Chiglet is right. It is quite heavily based on luck these days, but Manchester has just recently had an external recruitment drive so they do come up now and again. Be aware tho that most, if not all external posts will be ATSA1 in a simulator as a 'blip-driver'. But once you're in, there are many more opportunities that will appear. Just ignore all the crap about redundancies. They have been saying that for the last 15 years!
Depnding what sort of post you're applying for, eg Area, Approach or Airport, my advice is to learn as much as you can about the airspace (sector names, geography, even frequencies etc) and how ATC works in general. Do that and you should be in good stead for any interviews you get. The UK AIP is a good stop for up-to-date information too. Failing that, an ATSA I know was successful by playing on the London Control ATC simulator on her PC. It only costs about £30 and is really accurate in terms of appearance and functionality.
You can download the demo for free, and buy it if you wish from their site.
http://www.londoncontrol.com/
Good luck