Had a similar issue with MyHermes recently.
Sold a camcorder on Gumtree, and posted it via MyHermes, paying the extra cost to cover it up to its sale value (£50). At no point during the postage purchase did it flag up that the item I was posting was on their excluded list and/or that paying the extra cost to cover it to its value would be money wasted due to that list.
Anyway, buyer contacts me to say it hasn't arrived so I track it - it is tracked from the parcelshop to the local depot, then nothing more. Hermes look for it but cannot find it. I had to refund my buyer and then Hermes refused my claim for loss due to their extensive excluded items list.
Anyway, long story short, after reading on another forum, I sent them a LBA with the full intention of taking it to the small claims courts, and then miraculously, Hermes paid me back for the item and the postage cost as a 'goodwill gesture'.
Have a read on this forum for advice:
https://www.consumeractiongroup.co.uk/forum/183-postal-and-delivery-services/
The key to this is the excluded items list - its fair enough that they wont cover you for damage to an items, which could be arguable where it happened - but when they claim to deliver an item or lose it altogether, then the excluded items list should not be a factor.