I also agree that departments will pick the best qualified candidate. While living in the US, I've been looking for work in the UK. I chose to get a work permit just to make my application appear a bit more attractive. The work permit I have also allows my wife to work legally in the UK.
There are, at least, two types of work permits available. Brummagem has what I think is the most common type of work permit. That is, brummagem's UK employer worked with the UK Home Office (Immigration) to give him or her a work permit. That type of work permit applies only to a single employer. If brummagem decides to work for a different employer, then that different employer will have to request a work permit.
Another type of work permit uses the Highly Skilled Migrant Programme (HSMP). This is the kind of work permit I have. With this work permit, I can work for any employer in the UK. As I mentioned, my wife has the same option. This is a points-based application, which emphasizes education and income. If I remember right, there are 4 categories: Education level, income, previous experience living in the UK, and age. Younger applicants get more points than aged applicants such as me, since I received 0 points in this category. (Warning the next sentence may sting a little.) I'm 41.
You need 75 points to receive an HSMP. I think I received 50 points for having a PhD and 30 points for my income level. There's also an MBA provision that can give an applicant a similar number of points as having a PhD. Then, of course, there's the paperwork and other requirements. You also have provide 2 forms of very clearly specified evidence for every item you claim points for. I had to send the original copies of diplomas, official transcripts, copies of my payslips, and a letter from my university stating that the income I claimed was the income I earned. I think there's something about giving away a first-born child too, but I managed to get a waiver on that one, at least for now.
More information is at:
http://www.workingintheuk.gov.uk/working_in_the_uk/en/homepage/schemes_and_programmes/hsmp.htmlThere's also a link to an FAQ on the Web site above.
If that long URL doesn't work, you can search on the words "hsmp uk home office". Some results will appear for companies will complete your application for you, at a price. I don't recommend using these companies. Even though I made a couple of jokes about the process of applying, I also want to say that I found the forms to clear enough. I also found staff at the HSMP office to be helpful when I called. A final item I was impressed with is that I was told it would take 5-15 weeks to process my HSMP application. I received my approved application back in 4 weeks.