The lack of delivery is their responsibility, by law*, and not yours.
* Sale of Goods Act 1979 Section 20, 4) - the goods remain at the sellers risk until they are delivered to the consumer
I believe that is quite correct. If the sender didn't obtain a "proof of delivery" from the Post Office then they have no reason to dispute your claim.
However, this hasn't always been the case, and I'm posting this for the benefit of those who might remember a quite different legal framework :
Before "privitisation", the Post Office was a function of "The Crown" (hence the name
Royal Mail of course). And because The Crown was above the Law, (by definition, laws were passed BY The Crown), then it was enought for a sender to show "that they had placed the letter into the hands of the postal authorities" and that would
always lead to the assumption that The Crown, being above fault, would ensure that the letter was always delivered.
Purely an historical point, for your interest.