Earlier quoted context omitted.
> Isn't it some core legal principle that legislature can't do that, and always has the power to reverse itself? The problem with having supreme power to break any contract without consequences is that nobody will make contracts with you any more. For this reason, many states have (e.g.) clauses in their constitutions saying they won't. Of course, the legislature usually has the power to amend the constitution.
I'm moderately sure that the UK government has that power; we have no equivalent of the contract clause here. People still make contracts with the UK government.
The UK's Constitution is utterly terrible. In many places if you tried to implement something similar it would probably self-destruct very quickly. The UK manages to keep going with it (not brilliantly, but society hasn't completely collapsed yet) only because there is a strong democratic culture. That's my theory, anyway.