Public Funds have been allotted to protect tenants so they don't lose homes not make money. In some cases the housing element of Universal Credit is paid to tenants but doesn't reach landlords. There is no option to register a complaint. They are 'earning' UC benefit but not using it for rent.
You may be interested in these active petitions
If you are a tax payer please sign this. Public pay tax. Tax is used for benefits which is often given to help pay rent.