If your employee is from the European Economic Area (EEA) they have the right to work in the UK. If they are from a country outside the EEA they generally need to have a work permit. There are certain exceptions however, for instance Commonwealth citizens who can prove that one of their grandparents was born in the UK, and staff of diplomatic staff and their immediate families may be eligible to work in the UK without a work permit.
Please contact Work Permits UK on 0114 259 4074 for more information.