As of yesterday (8th of June), the new UK quarantine rules for arrivals to the UK are in force. The details can be found here.
The essentials of the quarantine rules are:
If you are a resident or visitor travelling to the UK on or after 8 June, you must:
You should read the legislation and then the Exemption Letter Guidance. The second attachment (which contains links to complete information on gov.uk) and Annex B (a list of exemptions) will help you decide whether or not your workers/contractors should qualify for an exemption.
If you think exemptions apply, you will need to produce a letter (see Annex A of the second attachment or draft your own) for the employee/contractor to show on arrival in the UK.
Please note that numbers 6, 26, and 37 of Annex B’s exemptions may be of particular relevance.