So in the case you are talking about, you are effectively talking about the Post Office, and the salesperson there who has taken your money as an agent for the insurer, correct? On this basis, the insurers/parcelforce will 100% decline the claim and you would stand zero change of overturning that, so I guess what you're actually left with is taking the post office to court, with any legal assistance you have have with your home insurance or similar. The prospect of success there would be so low I doubt any no win no fee would take it on for you. What would you be relying on, CCTV, audio recordings at the desk? You'll have nothing to prove you were told the wrong thing. And the terms and condition explicitly state the guitar contents of your parcel are not covered for enhance protection. Of course, there could be things in the package that are covered, the guitar case perhaps, string packs in the case etc so you have still been sold insurance for the right reasons, complicating it further.
Don't get confused with PPI and all that business, entirely different kettle of fish. Here we're just talking about straightforward exclusions in an insurance product that you're choosing to buy, and are only able to buy after you have confirmed you have read the prohibited list.
Best advise; don't use Parcelforce if you want you're guitars covered, and definitely don't on the basis that you can challenge them if something happens. You'll lose, sadly.