In order for you to terminate the tenancy prior to the expiry of fixed term or break clause you would have to show that the landlord has committed a repudiatory breach of contract which is so serious that it goes to the root of the contract to deny you all or substantially all of the benefit of the contract. Effectively this must mean that the property is unfit for human habitation.
Your difficulties with the boiler are, in my view, not sufficient to constitute such breach because you are still able to have heating/ho water. Were you to unilaterally terminate then the landlord would probably be able to sue you for the remaining rent up until the point at which he could relet the property.
You do have a right to a boiler in repair and you can enforce this by writing to the landlord specifying the disrepair and asking that he repairs it within 14 days otherwise you will pay for the repairs yourself and then seek to claim the expense from him using www.moneyclaim.gov.uk though I accept that this does not help in getting out of the tenancy.
If you suspect that the landlord would not sue you for the rent or would not realise that the boiler problems are not sufficient to entitle you to terminate then I suppose you could take a risk and terminate in the hope he would not sue, but this seems too big a risk considering that you have the option of a break clause coming up.
Better in my view, to make sure you exercise the break clause OR perhaps offering to leave early and pay a fraction of the rent for time remaining until break so he can attempt to relet immediately to make a small profit.
Obviously though you should write to him as above stated in respect of the disrepair.
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