I mean there are definitely risks, and we have a lawyer keep us right, but I've never heard him specifically mention a 10k fine.
Depends what the joiner was doing: essential work is described as being necessary to the maintenance of a building. Which could be anything: window frames, fixing fencing, flooring, shelving, etc. That's why 'essential' is a bit of a joke, and why building sites are being shut down by positive cases.
In some EU countries you need a negative test to get in a site. Here we're sending guys across the counties in shared vans to work on replacing 8x10 bathrooms together.