A tenant can only terminate earlier than the fixed/break clause in the event that the property is unfit for human habitation. This would be a breach of contract by the landlord called a repudiatory breach, meaning that the breach is of so serious a nature that it goes to the root (ie. the heart) of the contract to substantially deny you the benefit of the contract.
To safely do this you would have to have documentary evidence of the state of the property demonstrating that it is not fit for human habitation. Ideally this would be in the form a a declaration made by the environmental heath department of the local authority. I note you have been in contact with the authority but if you have not actually spoken to the environmental health department then I would do this immediately and ask that they make an emergency inspection.
If the landlord does not accept that it was unfit for human habitation then will first dispute over the return of the deposit and then it would be for him to issue litigation proceedings against you to sue you for the rent he would have received from you up to the time at which the tenancy is terminated.
If you were minded to terminate claiming a repudiatory breach then I would say so in a letter to the landlord sent by registered post listing all the matter which contribute to the state of the property (lack of electrical certificates, gas certificates, disrepair) and at the same time noting your efforts to get him to make appropriate repairs and that these have been left unressponded. State that if he seeks to resist the the return of the deposit or the legal termination then you shall defend any claim he makes against you as well as make concurrent complaints to the Council, trading standards and the Association of Residential Lettings agents (if they are a member).
That he has exercised the break clause may help you - he may actually want to get you out early and may accept that you will not pay rent. This would be a surrender of the tenancy and you would have to get this in writing from him. I would contact him to size him up on this option.
I assume the deposit has been registered with a tenancy deposit scheme, if it has not then please let me know and I shall advise further
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