Obtaining work permit before getting job offer?

HELP!
While applying to several jobs online, I'm almost always encountering the following question:

Are you authorised to work in the location(s) shown above by being a citizen of that country or region, or by holding a valid permit to work in the location(s)?

How can I obtain a valid work permit when I don't yet have a job/employer who would do that for me? I have asked this question so many times on several different job sites but no one has been able to respond. I realize I need a work permit to work in London but I can't get one until I have a job offer. Help with this process, please!!
 

Hi Katspee, I've responded to your PM.

You have to have a work permit through the employer.  You have to have the job then the employer gets the work permit.  YOu can get a Highly Skilled Migrant Program visa which allows you to work wherever.