I happened to take a look at the Post Office travel insurance's policy wording and it seems fairly generous. Notably, it explicitly covers the "face value" of Avios - I'd have said the purchase or subscription cost would be a reasonable definition for this. It also gives you £50 (or lounge access - not that most FTers will need this) if any flight you're on is delayed by 1 hour. With the number of delayed BA flights you could easily be quids in on that front! You do have to register flights on their portal at least 24 hours beforehand to be eligible for this.

I did a quote and it came out at around £140 for an annual multitrip policy with zero excess and including winter sports. So on the same order of magnitude as the Nationwide policy, but obviously not including the breakdown or mobile phone cover.

The only downside I can see is that flights etc. have to be delayed due to one of the specified reasons (industrial action, bad weather, technical fault, or shortage of crew) for the missed departure/connections cover to kick in. So there are some edge case scenarios where you wouldn't be covered, and you'd no doubt have to get written confirmation of the reason in order to make a claim.

But it does show that there are still underwriters who cover risks/costs that the likes of Aviva don't.