With the gas boiler there is no such thing as “likely to be unsafe”. It is either safe or not. Although you do say unlikely to be unsafe, I think you mean likely to be unsafe. Please confirm.
Whilst I am aware of your concern about it exploding, a Gas Safe engineer would seal it off if there was any danger of that so that really isn’t part of the equation.
There is no claim for “may stop working” although you do have a claim under section 11 of the Landlord & Tenant Act for landlords breach of repairing covenant in respect of the water leak and the downtime.
You are entitled to compensation but you are not entitled to end the tenancy early or for a rebate in rent unless the landlord agrees.
On the other hand, if the Environmental Health Officer at the local authority declares the property unfit for habitation, you can walk away with impunity. It’s unlikely you would get that notification.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have