You’re overthinking the issue. The laws not authored but enforced in the U.K. are done so with the express authority of Parliament. Treaty ratification is literally “passing an Act of Parliament that implements the treaty in U.K. law.” Parliament can also delegate legislation authority to other bodies. It can also revoke that authority (within the U.K.) either in part or in full.

This has actually happened on multiple occasions: Brexit being one that removed European institutions’ right to set rules that the U.K. must comply with. It’s also an example of where this isn’t free: the government agreed to a bunch of rules right back because cutting ourselves off from Europe would have obliterated the economy.