So I've done the same when I bought my last house(conservatory built 20 years ago, seller couldn't produce the correct paperwork for it), but the issue wasn't the planning permissions per se, it was that the original builder of the house(Belway) hasn't given permission and they reserve the right to do so for 99 years after construction(even though the house is a freehold now, the entire estate is still Belway's leasehold). So the insurance was against that as well.

Like, are the chances of Belway complaining and landing us in legal trouble close to zero? Yes, they are, but like said above - it was £50 and I got the seller to pay for it. When we sell the house I'll do the same just to avoid any bumps in the process because of it.

How do you make sure that the possibility is actually non-existent?

If it's only a small one-time payment, then it probably makes sense to pay someone that much just for doing the work to verify that it's not going to happen.

Planning enforcement is 12 years from the date at which it became obvious that building works had been completed. So if you build a castle inside a agricultural shed and take the shed down after 12 years, the timer starts after you take down the shed. Also, there are many caveats and Planning law is complicated and often quite vague in Britain, so you should take advice on your specific situation.

Those indemnity policies are so common that any related admin work has to be included into fixed fee conveyancing deals.

In fact I suspect that these policies are so common partly because of they save solicitors' time and efforts, not least when they are on a fixed fee: They don't spend time looking into things they just suggest indemnity policies so they are covered and move on.