Owners of commercial premises (including holiday lets, guesthouses, bars, restaurants, etc) should take all reasonable precautions that they do not knowingly accept customers who are not meant to travel to that location.
Accepting a booking from someone known to come from a restricted-travel area, who then brings Covid-19 to your premises, might restrict your ability to make a claim against some aspects of your insurance cover. For example, a person from a Tier 3 location should not be travelling outside that place for leisure purposes.