Capital punishment for murder was abolished in 1965, and the last people to be hanged for murder was in 1964. This remained unused until Britain signed up to the European Convention on Human Rights. Either way, the Human Rights Act 1998 was an Act of Parliament passed by the UK Parliament and not subscribed to us by the EU. The ECHR is technically 'binding', but again, Parliament can ultimately opt out of it and indeed, the EU as a whole.
It's not a hypothesis, this is how the UK constitution works. In the passage of laws in the UK, the EU plays no part in the process of our lawmaking, and there are no EU laws on our statute books -- every EU law and legislation or treaty that applies in the UK was ratified, or passed by an Act of Parliament. The Human Rights Act 1998 is an example of that -- which Parliament was planning to repeal even before Brexit. Parliament has subcontracted sovereignty.
Your claim the UK cannot physically pass laws that contradict EU law but at any point of analysis, the EU has no physical input in the passage of our laws. The EU laws and legislation we are 'bound' to was with Parliament's consent.
In a globalised economy, the sovereignty of national governments is being eroded. If a country has high levels of taxation, multinational corporations relocate, just as Dyson has done this week. Free trade deals also subcontract sovereignty, look at the impact of the North Atlantic Free Trade Agreement (NAFTA) on Canada, their Government has been taken to court over their environmental regulations and their healthcare systems, and something like 70% of claims has been bought against Canada. Governments just don't have sovereignty of pre-WW2 national states. Assuming we got free trade deals across the globe, there will be regulations the Government has to stick to, in essence, we're still subcontracting sovereignty, in or out of the EU.