Some sweeping statements here Teddy123.Fantastic informative post.
The fact that P&O have a clause that excludes compensation for consequential loss does not legally 'allow actions' if it is unfair to the consumer, no matter if the consumer accepted the terms and conditions when booking. The consumer did not have the opportunity to negotiate the terms of the contract so the contract, to be valid, has to be equally fair to both sides. I refer you to the Consumer Rights Act 2015 (formerly the Unfair Terms in Consumer Contracts 1999).
You state 'Any claim would depend on proving the relevant clause were invalid'. You certainly do not have to prove that. There is a procedure by which you claim recompense for loss.
1) You write to the company stating your claim with supporting evidence.
2) If the reply is unsatisfactory you write a 'formal letter of pre court action' outlining your case and the amount you are claiming, and also offering them mediation to settle the matter if they so wish.
3) It there is no resolution ( and in many cases the company will settle if the claim is fair ) then there is a simple application form for recompense which you send to the Claims Court. Details of that are available on line.
It is up to the Court to consider whether you have been treated unfairly and decide the outcome. If the company fails to defend the action then the applicant wins by default.
I have used the process (against P&O) and they settled after the formal letter. The cost of it is minimal, in my case 3 first class stamps.
It rather appears from recent posts that they are considering compensation for incurred losses because of their action, so they will know that they are on unsafe ground if they hide behind their Clause 42. The last thing they will want is a successful court judgement against them.
Lets hope everyone concerned reads this and acts accordingly.