Ok, I actually know something about this. My sister tried to get a job in the UK a few years ago. The company wanted to hire her but couldn't get through the red tape.

Basically you need a work visa (obviously) but it's one of those catch 22's. You can only get one if there is someone willing to hire you but most places won't hire you unless you have one. That leads to the second problem and the red tape. For someone who is not a citizen of the UK to be hired the company hiring them has to be able to prove that there was not a qualified canidate that they could hire who is a citizen. Wait time is usally at least 8 months but can take 1 1/2 years. If you have the time then good for you. Get in touch with people in the business over the net and start networking. They can give you the details.

Best of luck with your erasmus (test?).