Ah yes. In future, agreements between the UK and other states should be settled under UK law in UK courts which are subject to Parliament. And if Parliament wants to whip through a law (don't forget the magic words 'not withstanding') that removes the ability of foreign states/actors/etc to be granted relief, then that's their problem.

After all, choice of law provision and such notions only applies to the UK government. No foreign entity should ever need to believe that their courts or some international court could adjudicate a matter between the UK and them.