A potential tenant has some incentive. If there was a public record of which properties were rent controlled, as a tenant looking for property and I ran into on

A potential tenant has some incentive. If there was a public record of which properties were rent controlled, as a tenant looking for property and I ran into one I thought was sublet, I could compare it with the list and report it. Basically it would become hard to advertise them because it would be easy to check. TBH, you could make it illegal to advertise property that was being sublet and put the onus on Zoopla/RightMove/whoever to do a quick API check for an address. You're right that if it was the cheapest way of getting property then tenants probably wouldn't report. But if the whole thing is making property more expensive then I definitely would. Recently on Reddit there were some screenshots of a ground-floor 'studio' flat that was entered through a window. It didn't have a door and was still quite expensive. That's illegal according to UK rules, and lots of people on Reddit happily reported it.