A UK Work Permit is a document issued by Work Permits (UK), a part of the Department for Education and Employment; it gives your employer permission to employ you in a specific job at a specific location.
A UK Work Permit is not a permission for you to take up work in the UK. The permission for you to enter and work in the UK is conferred by a UK Immigration or Consular officer who, following the issue of a Work Permit, may issue you with a suitable visa. The visa is an endorsement stamped in your passport.
Even if you are already working in the UK on a Work Permit, if you want to move to another job, your new (prospective) employer must obtain a new Work Permit before you can change jobs.