A UK Work Permit is a document issued by the Work Permits (UK), a part of the Home Office Immigration and Nationality Directorate; it gives you permission to employ a specific person in a specific job at a specific location.
A UK Work Permit is not a permission for the employee to take up work in the UK. The permission for an employee 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 to the employee a suitable visa. The visa is an endorsement stamped in the employee’s passport.
Even if an employee is already working in the UK on a Work Permit, a new employer must obtain a new Work Permit before the employee can change jobs.