Insurance purchasers need to distinguish between:
* Third party liability. Designed to pay out if someone sues you. Most people seem to get this via CTC or BC or an affiliated club.
* Theft, accidental damage etc. Designed to compensate you for the cost of the bike. Most people have this to some extent via a household policy - a lot seem to pick their household policy specifically for this cover. It comes under the contents section, so it is available to people who rent rather than own. It is also available standalone, but that seems to be thought of as an expensive route.
* Personal accident. Designed to pay out if you get injured. Not many people have this, and not many companies offer it specifically for cycling.
* Breakdown cover. Designed to get you home after a breakdown or incident. Few people have this; at least one railway company offers it to ticketholders.
* Travel insurance. Designed to pay out in the event of injury or financial loss while on holiday. A standard travel policy may in some circumstances pay out if a bike tour has to be abandoned.
* Legal expenses. Designed to pay for legal fees if you need to sue someone else. Most people seem to buy a "no-win no-fee" policy after any incident if their lawyers think there's a reasonable prospect of success.
People need to do their own research. What's best for one person may well not be best for another. Prices can vary wildly. Different companies have different policies and different approaches to settlement. And read the wording of the policy carefully, with advice if necessary!
For the record - I have CTC membership, so third-party liability cover; theft and accidental damage via my household contents section; annual travel insurance which doesn't exclude cycling holidays. I work for an insurance company, which provides some of those products.