The common law provides that where a tenancy is determined, all sub-tenancies automatically end unless the superior tenancy has been determined by surrender.
Termination of head tenancy: effect on assured [shorthold] tenancy
Section 18 Housing Act 1988 nullifies this rule with assured [shorthold] sub-tenancies lawfully granted by providing that any such assured sub-tenancy shall continue where the superior tenancy is determined.
The person who would but for the assured [shorthold] tenancy be entitled to possession of the dwelling-house becomes the new landlord [s.18(1)]. This rule will not apply if the person who would be the new landlord is entitled to rely upon one of the exclusions in sch.1 Housing Act 1988 (e.g. rent over £100,000 etc.) [s.18(2)].
A landlord who has taken care to select a tenant by proper referencing and verification of suitability is unlikely to allow that chosen tenant to sublet, assign or transfer the tenancy to another without the landlord’s permission. In the past, tenancy agreements always tended to prohibit subletting or assignment.