If you do not licence a HMO
It is illegal to operate a House in Multiple Occupation without a licence (unless it does not need a licence).
If you do, you could be fined up to £30,000 by the courts and tenants will be able to reclaim up to 12 months of rent.
If a HMO should be licensed but is not, the landlord can’t serve notice to quit on a tenant until they have licensed the property.