As the answers said above. They have been open and direct, and stated "these things won't be covered". You went ahead anyway. They have a perfect right to decide what is and isnt taken and on what basis its taken (if it is taken at all). They can decide a long or short list on a whim. That puts you at a disadvantage to start with.
As to what you can do despite that, you need to give a bit more info. I have to say, the chances look slim and grasping at straws. We can try, though.
Did you tell them specifically it was a TV or high value? (And if so did they point out the possibility of exclusion?) Did they ask? (Or warn of the possibility?) Did they make the terms of carriage/insurance clear (via a link or otherwise)? Was there a "I agree to the terms of carriage" box which you might have looked at? Objectively, was it made reasonably clear to a customer to whom it mattered?
One aspect I'd check is whether it was implied that P2G don't compensate for these items and therefore if you need carriage with compensation, you will have to pay for the insurance on top.
A second aspect is that the insurance may technically be separate (or presented as separate/separable in law) from the actual carriage. For example, check who provides or underwrites the insurance and if its actually a policy, or a separate policy, or just an extra limit they offer if you pay more. It may be that even if P2G say they won't compensate you, the policy provider has not made such a statement and arguably remains bound to compensate when P2G isn't.
A second tack here is that you might be able to argue two prongs of a technical point, something like this: under insurance law, if something isn't material to the insurance then it can't be used to block a claim merely on technical/"small print" grounds. But if it is material then it's up to them to ask for it or state it needs to be disclosed, not for you to volunteer it. If they said or implied that additional insurance could cover the package, then you might argue you relied on that statement and understood it to mean P2G don't compensate (or that you didnt understand it to be relevant whether or not P2G excluded it or not) but separate insurance did, because it doesn't say anything to the contrary. To the response that its excluded, you reply that you didn't read the long list of things P2G don't cover themselves, because you knew it needed additional insurance and in good faith understood that to cover it: "it didn't say otherwise". And, indeed, it doesn't say otherwise - there is no clear statement that your expensive items aren't compensated even by the insurer (or by a separate insurer), or even if separately insured.
You might also check if anything else covers it. Household insurance maybe?
I don't think unfair contract/general consumer law helps much (other than perhaps "was it clear enough" and also the separate question "was it clear enough to someone who had decided to buy insurance for their item that the insurance wouldn't be claimable either"), because nobody made you choose them, and they were otherwise open about their exclusion list.
