You would be right in your own fantasy, except in the real world the UK made it UK law by signing that treaty and ratifying it. Sure, nobody else on the planet cares about that little aspect, because as you righthly point out, why would anyone else give a shit about your domestic laws. But you would be breaking UK law by breaking the international treaty. Oh, here's the misunderstanding... you think that you can change an international, bilateral treaty with a unilateral domestic UK law amendment? Uh, that's not how it works, buddy. You agreed to abide by the ruling of that international panel. You signed off on it. You ratified it. You wrote it into UK law. You are bound by it. And the only way to change it is to get the EU to agree to that change. Bilateral. Means two parties have to agree to it. Nobody forced the UK to agree to this. Nobody forced the UK to sign it. Nobody forced all the illustrious MPs to ratify it. That's all your own doing. So, yeah, you'll pay up. Why? Because your own Supreme Court will order your Government to pay up. Or, you know, follow the ruling of the panel.
How long do you want to play this? I can go on forever, mate. See, I don't even have to think hard about it. All I need is to know how law works. And unfortunately for you, I seem to have a better working knowledge of the UK legal system than you do. Bit embarassing, I know. But rest assured, I feel even more embarassed, cos I know jack shit about the UK legal system. But there you go, it's enough to school you. Again.
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Be that as it may, it's not going to be an all UK board of Brexiteers falling down in a fit of laughter because they read EU in the title. That was my point.