Race Insurance is almost always liability insurance, not cancellation insurace. I do not know what cancellation insurance would cost, but I know that it would end up on your entry form price for every single race, and in the end you would pay more for that than accepting the odd £20 here and there. After all the insurance company wants to make a profit out of it. If they reckon 1/1000 races are cancelled at a cost of £120,000, then they will charge £250,000 over those 1000 races.
The GNR and FLM are other races that would not refund you anything if the race was cancelled. UKA advise that race cancellation for reasons outside of the organisers control mean that the organiser does not have to give a refund. E.g. The police can close a race at any point. Imagine lining up at the start line and being told the race was cancelled. It could happen is there was a severe accident on the course. Having to give a refund could bankrupt an organiser - which might be an individual, a charity, or a local running club.
Our T&C are slightly different - we won't refund you, but you do get a voucher for another of our races for free entry (named race only and unattached levy still needs to be paid (currently)). If you can't make the other race, then there will be a smaller discount to another of our races within a time period.
We cancelled one race and it cost us dearly (weather). We also had to think what we could do at the time - we didn't have a policy until it happened. Most runners were very understanding... Obviously times have changed. Our race had 600 people in it. The W0.5 has 6000 - more problematic.