It is an implied term of every assured shorthold tenancy that the tenant cannot sub-let or assign the tenancy without the landlords consent under section 15 Housing Act 1988. However, that right is lost if a “premium” is taken. A premium is defined as (amongst other things) if a deposit is taken which is greater than 1/6th of the annual rent (greater than 2 months worth of rent).
Therefore landlords (and their agents) should not take anything greater than the equivalent of 2 months rent as a deposit ever. Effectively the tenant could assign or sub-let whenever they liked if you were to do so.