A UK Work Permit is a document issued by the Overseas Labour Service (Work Permits (UK)), a part of the Department for Education and Employment; it gives a UK employer permission to employ an overseas candidate in a specific job at a specific location.
A UK Work Permit is not a permission for the candidate to take up work in the UK. The permission for the candidate 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 the candidate with a suitable visa. The visa is an endorsement stamped in the candidate’s passport.
Even if the candidate is already working in the UK on a Work Permit, if they want to move to another job, their new (prospective) employer must obtain a new Work Permit before they can change jobs. However, this process is generally straigntforward