Yes, but your original quote from Royal Mail;
"Unfortunately we can't accept claims where the problems were caused by another postal operator or other third party"
gives them a get-out clause should they wish to use it, therebu slope shouldering the problem back onto us. That's not a 'service'.
It shouldn't be like that. They should offer a service, for which we pay a fee, after which we shouldn't have to worry about a thing. That's what insurance is all about.
Seems to me that Royal Mail would like to have their cake and eat it too, when it suits them.