Although a landlord can re-take possession if it is obvious that the tenant has abandoned the property, in most cases the landlord will need to obtain an order from the court. Evicting a tenant without a court order is a criminal offence (with very few exceptions) as detailed earlier.
If a tenant ceases to occupy an assured shorthold tenancy as their only or principle home (i.e. abandons the property and lives elsewhere), without more, the tenancy does not end – it simply ceases to be an assured shorthold tenancy and becomes a contractual tenancy.
As a tenancy remains in force, a court order is required.
When debating if there is abandonment, the question is not “is somebody living here?” – the correct question to ask is “does the tenant intend to return at some future date?”