Setting up a tenancy
If the owner of a property agrees to allow someone to stay there in exchange for rent, they immediately create a legal relationship of landlord and tenant.
This is different if the owner also lives in the property.
Almost all new private tenancies that started after 28 February 1997 are Assured Shorthold Tenancies. These were set up by the Housing Act 1988 and subsequently modified by the 1996 Housing Act. The Housing Act 1996 sets out how to start and and Assured Shorthold Tenancies.
Even if there is nothing in writing, once someone moves in, a tenancy exists. The same basic rules apply to ending the tenancy as if it was started with a written tenancy agreement.