All such policies exist solely to make a profit for the company that sells them.
In my opinion, one should buy insurance only for risks that result in large costs you can't cover yourself, e.g. your house burning down. If you can afford to pay for your boiler to be repaired then on average you're going to be better off putting what you would have spent on premiums into a building society account that you can draw on when needed.
If you do go ahead, make sure you understand any exclusions and what happens if the insurer decides that the boiler is beyond economic repair.