• Breach - tenants in breach can normally be evicted via court action
• Unfortunately court action is notoriously slow..
• Tenants who know their Housing rights and the difficulty of landlords enforcing breaches, may take advantage, resulting in landlords paying for what they are already legally entitled to, but are unable to enforce their rights within a cost effective time.  E.g. tenants who demand to be released from their outstanding arrears of rent in order to agree to an earlier landlord application for a possession order.  I.e. by the time landlord secures eviction, they would already have lost more in e.g. unpaid rent, than the cost of them remaining but paying no rent meantime.  The landlord in a lose-lose situation, would ultimately win the case but lose the interim rent over a protracted eviction due to court delays and, from a tenant man of straw, might never receive any arrears via an unenforceable CCJ.
• A difficult tenant forming part of an HMO might result in housemates determined to leave due to the difficult tenant's unacceptable behaviour.  It might make it prudent for the landlord to pay the problem tenant to leave in order to avoid having only one tenant paying rent in say a four bed house thereby reducing the landlord's realisable rent by three quarters!  Better to lose only one quarter rent initially, with the prospect of finding an ultimate replacement, than lose three quarters rent with no realistic prospect of ever replacing the fleeing tenants!

See also

• enforcement
• Section 21 Housing Act 1988 Accelerated NTQ Fixed term tenancy
• Section 8 Housing Act 1988 NTQ periodic term tenancy also protected tenancy