The basic position is that you are only entitled to terminate the agreement if the property is unfit for human habitation. It it is then this would be a repudiatory breach of contract - a breach so serious that it goes to the root (ie. the heart) of the contract so as to deny you the whole or substantially the whole of the benefit of the contract.
The issue is whether it is in fact unfit and if you can prove this. If you suspect it is then I would contact the local authority's environmental health department and ask them to make a inspection. If they find it unfit then they will declare it as such and you can ask them to provide this in writing. This would be sufficient evidence for you to terminate confidently.
If they do not declare it as such then you would have to prove this yourself. If you do not prove it to the satisfaction of the Court then you could be liable to pay the rent up to the point at which the Court decide the property could be re-let by then landlord (they are under a duty to mitigate their loss by reletting).
If you doubt this can be proved you can require them to carry out repairs, you have a right to a property in repair. You would write formally to them setting out the repair and asking for them to make the repairs within a reasonable time (ie. 14 days) failing which you would state that you will pay for the repairs to be made yourself and then seek to claim the expense from them by issuing a claim against them.
You could do this via www.moneyclaim.gov.uk.. It's very straightforward and easy to use.
The agreement is enforceable and unless you can terminate on the grounds of uninhabitability then you will have to enforce your rights to repair in the above way.
If you can get them to agree that the property is unfit now by sabre-rattling and threatening to call environmental health, or perhaps negotiating a lesser rent for the time being until the repairs are carried out then this may also be a solution.
Get everything in writing (eg. email).
If you withhold rent in respect of the above then this may entitle the landlord to terminate the agreement, they would then sue you for rent and would probably get judgement on an amount equal to the rent they would receive until the property could be re-let.
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