Why shouldn’t they be able to continue their work (assuming, when you mention “gangmasters”, that you are referring to fruit and vegetable production)?
Among the exceptions to the rules on leaving your home:
“to travel for the purposes of work or to provide voluntary or charitable services, where it is not reasonably possible for that person to work, or to provide those services, from the place where they are living;”
And exceptions to gatherings of more than two people:
“where the gathering is essential for work purposes,”