It is a common misconception on the part of both landlords and tenants that once a Section 21 notice has been served and the 2 month notice period has expired that the tenancy has been brought to an end.
This is not the case. The tenancy can only be ended either by the tenant surrendering the tenancy or a court making a possession order.
Landlords who act as if the tenancy is over without obtaining a court order or who attempt an eviction after the expiry of notice could easily find themselves in trouble. Harassment and illegal eviction are both crimes. A tenant in this situation facing an illegal eviction should call the police first, not a lawyer. If a tenant is evicted illegally and the police do not help then the tenant will need a lawyer to obtain an injunction to get back into their home.