I think you will find that most companies offer this as an optional extra to buildings/contents cover. With my insurer (AXA) it costs an additional £39.99p per annum.
So if you have decided you want it, compare the AA cost with your existing insurer, because it is usually the case that bundling it as an optional extra is cheaper than establishing a separate policy (because insurers have fixed costs with just operating each policy).
Whether you think it is worth £40 for peace of mind is up to you and you will undoubtedly get different answers, because it's a personally thing.
Your decision may depend upon:
if your home is well-maintained and relatively new, so less likely to create such an emergency
whether you know enough to deal yourself with immediate emergencies. The thing most people rightly fear is water flowing - but if you know how to turn off at the main stop-valve, it may not be an issue
how resilient you are to waiting a couple of days to get a tradesman at 'non-ridiculous' call-out rates.