The law in the UK literally disagrees with you about how this works. In fact our Unfair Contract Terms Act (see that name? Turns out even the people who named this act of parliament disagree with your idea that if it's legal that makes it fair, they figured if it's unfair let's make it illegal) says that if you "freely" agree to these unfair terms they still can't work.

You can see this principle at work in Britain's short lease (as distinct from a long leases of say 99 years) residential property laws too. A bunch of potential terms for such leases are defined to be unfair. Some landlords - after this change - went to court and said "well, but my tenants signed to say they knew the terms were unfair and agreed anyway - see, it's right here on page sixty of the contact" and judges told them where they could stick their unfair contracts. So sure enough those terms went away.

As to how you tell, courts already know this. If party A wrote the contract and party B's options were to agree or GTFO then party A has an advantage. Its normal contract law - oh, McDonalds signed this lease, but Tiny Co wrote it, so Tiny Co were the ones at advantage and anything unclear must not be construed to their benefit.

It is more than plausible to abuse the legal system in ways that are (obviously) legal. If that claim is not what you are disputing, what is?

So it would be interesting if downvoters could, by way of reply, articulate how it is that contracts which are legal in all respects including all of the terms and the free will of those entering in to it, should be handled when they are deemed not to their liking after the fact.

If a large enough group of outsiders to the contract don't like the contract and can change the terms after the fact, wouldn't that undermine the confidence for people to enter in to them in the first place?