I think runningkeenster was referring to English Common law, which applies to contracts. There's the idea that if it was impossible for a party to deliver whatever the contract was about, then they don't attract a liability for that in law.
So if you book a flight, and it's impossible for the airline to fly you a a result of the law changing (for example) they aren't bound to complete the contract, and a court cannot compel them to. Depending on the T&Cs, this also means you won't get your deposit - or complete fare - back.
I'll (re) post this
en.wikipedia.org/wiki/Frustration_of_purpose
as it's as good as you'll get from Wiki. I'm not a lawyer, nor do I play one on TV. But I rub shoulders with a few from time to time - enough to know that any action against an airline that wasn't able to fly because of legal issues outside their control would be defended on the grounds of frustration in the first instance. Unless there's a more appropriate statutory defence.
Worth noting that if you can't take up a hotel reservation because you couldn't get there, you probably won't be able to claim consequential damages either.
Personally I don't have a dog in this fight, as I'm not planning on going anywhere privately for a while.
Of course the government can just pass a law relieving airlines of any liability once we're out of the EU. In fact, they may not even need to do that, as they've already got that power thanks to the Henry VIII clauses they slipped in when we were looking elsewhere.