@amy93 I did read somewhere that Royal Mail are having to step in to deliver some in order to clear the backlog, so it could be your new passport!
They switched to courier to speed the service up and make it more secure – but clearly the contract went to the lowest bidders, who obviously can’t handle the quantities involved (OK post COVID this is obviously adding extra pressure) but it’s not as if a passport comes in a large box requiring lots of vans etc, you can probably get tens of thousands of passports in a van, there really is no excuse for the poor service from TNT.