Your contract with the tenant must include what should be done with their belongings at the end of a tenancy and explain by which and what you can do with possessions left or abandoned in your property.
When a tenant leaves possessions, serve an S21 that states how long you are willing to wait for them to collect their belongings and what you will do if they don’t. If a tenant fails to collect, the belongings are considered as abandoned and you can do what you intend to do to them.