There's a determination just out that speaks directly to this (TR0001066, an IRES REIT case, February 2026).
Shared house, let as one dwelling. One original tenant (call her A) remained. Two others were added, fell out with A, and left in October 2024 without paying their share.
IRES served an arrears warning and a notice of termination on A for the whole house arrears and went for possession.
They lost the notice, and the reason is a piece in the jigsaw on this thread's topic. When the two left, the agent held onto the returned keys, so A couldn't re-let the empty rooms to cover the full rent. The Tribunal held the contract was effectively frustrated, because IRES's own conduct stopped A from filling the rooms, she could not be held liable for the arrears the empty rooms generated. Notice invalid.
Two things fall out of that, and both matter here.
First: the rent wasn't due while the landlord disabled her ability to earn it. The "one dwelling, one tenancy / joint and several liability" rule everyone assumes is purely landlord friendly has a hard limit: yes, the remaining tenant is in principle liable for the whole rent - but not where the landlord's own actions prevent her from generating it from vacant bedrooms. IRES tried to turn co-tenant departures into a whole-house arrears termination.
Second: and this is the quiet implication - it confirms the landlord controls who joins the tenancy. The whole frustration finding only works because A couldn't just install replacements herself. If she could put anyone in the empty rooms on her own say-so, withholding keys wouldn't have stopped her, and there'd have been no frustration.
The reason the keys mattered is that filling the rooms needs the landlord's consent to add a new tenant - the replacement joins the existing tenancy by assignment, which the landlord has to accept. So the landlord holds the gate on replacements. Which cuts both ways: it's a power (you decide who comes in), but it's also a trap - sit on that consent (or on the keys in this case) and you can't then bill the sitting tenant the rent for the vacancy you caused.