Its a general principal that you cannot contract out of law. The policy is a contract, between the insurers and the insured and as such is subject to UK law, it says this in the policy document.
Permission can be given retroactively, " You didn't ask, but I dont mind you being in there". So there is a grey zone. Also trespass is a civil matter not criminal law.
There also the CROW argument relative to caves on access ground.
One has to remember that the policy is a standard Public Indemnity policy that has been individually tailored for the purpose
As as absolute answer to your question I would have to say "No". Especially if it goes further than trespass and involves forced entry. NAMHO events require the organisers to ensure that relevant permissions are obtained.
Slightly off topic, but there is nothing to physically prevent anyone entering most sites on the CAL schedule, but from the above its easy to see what has been gained from the CAL agreement. NAMHO 2013 saw trips cancelled where permission could not be obtained. NAMHO 2019 is a different ball park.
Hope that all this helps.
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