Please see the following excerpt from guidance for tenants under the Tenant Fees Act 2019:
"EARLY TERMINATION FEES
Q. Can a landlord or agent charge me if I want to leave a tenancy before the end of my fixed-term or the end of my notice period? A landlord or agent can require you to make payments in connection with the early termination of the tenancy if you have requested this, but there are restrictions on what can be charged.Generally, the costs charged for early termination must not exceed the loss incurred by the landlord (usually the loss in rent resulting from your decision to leave and/or the costs of re-advertising or referencing), or the reasonable costs to the agent (such as referencing and marketing costs). If a landlord or agent agrees to your leaving early, they can ask you to pay rent as required under your tenancy agreement until a suitable replacement tenant is found. This is because you are liable for rent until your fixed-term agreement has ended or in the case of a statutory periodic tenancy, until the required notice period under your tenancy agreement has expired (if no replacement tenant is found during this time). However, a landlord is not able to charge more than the rent they would have received before the end of the tenancy.If a landlord agrees to terminate your tenancy early, you should make sure that this is clearly set out in writing. It is good practice for a landlord or agent to agree to a reasonable request to end the tenancy early. Where this is agreed to, landlords and agents should consider on a case-by-case basis whether it is appropriate to charge an early termination fee, for example, whether there are any exceptional circumstances which require the tenant to leave early. However, they could reasonably charge a fee to cover any referencing and advertising costs that they have incurred because of you leaving early, but they should be able to provide evidence to demonstrate these costs.Please note: a landlord or agent should not require you to pay any charges in this circumstance if you are exercising a break clause in your contract which permits you to leave before the end of your fixed-term (if you have given notice as required by the terms of your agreement). Q. What can a landlord or agent charge if a replacement tenant has been found? A landlord or agent may be more willing to let you leave early if you offer to help find a suitable replacement, as this is likely to reduce the up-front costs. Where a suitable replacement tenant is found and the landlord has agreed to an early termination of the tenancy, the landlord or agent can only charge you rent until the new tenancy has started. If a landlord or agent does not stand to lose any rent because of your decision to leave, they are not permitted to consider lost rent as partPage 72 of 81 of any fee charged for early termination. The landlord or letting agent could reasonably charge a fee to cover any referencing and advertising costs that they have incurred because of you leaving early, but they should be able to provide evidence to demonstrate these costs.Q. What should I do if a replacement tenant has not been found? If there is no replacement tenant and the landlord or agent insists on you paying rentuntil the end of your fixed-term agreement, we would encourage you to continue paying your rent monthly (or as required under your tenancy agreement), until a new tenant is found. You are not required to pay the outstanding rent amount as a lump sum unless you still agree to terminate the tenancy and agree this with the landlord.Q. Can I sub-let a property as an alternative to terminating my fixed-term agreement early? You should not sub-let a property unless your tenancy agreement allows this and this has been agreed in writing by the landlord.If it is not appropriate for you to sub-let the property, we would encourage a landlord or agent to let you leave the tenancy agreement early provided that a suitable replacement is found. "
The above is from here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/819635/TFA_Tenant_Guidance_190722.pdf
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