Where the tenancy is registered at the Land Registry and the contract provides that it will terminate on non-payment with immediate return of the property, termination operates automatically. It is enough to serve a demand on the tenant, judicially or through a notary, at the address designated in the registration. If there is no reply within ten working days, or the reply accepts, the record of service is title to cancel the tenancy at the Registry.
It is a provision of the Urban Tenancies Act that is rarely used, and it is decided when signing, not when the arrears arrive. Which is why reviewing the contract before signature is usually worth more than any later proceedings.