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.