Retrospective criminalisation of formerly lawful acts is generally prohibited in legal systems that respect the rule of law. The European Convention on Human Rights, Article 7, explicitly prohibits retrospective criminal penalties. The principle extends by analogy to civil liability: you should not find yourself financially liable for acts that were lawful when you committed them based on moral standards developed later.
The reparations claim requires implicitly retroactively treating the slave trade as an unlawful act creating civil liability. But it was not unlawful at the time. Parliament could, in theory, pass legislation declaring it to have been unlawful and creating liability, but such legislation would face serious legal challenge and would set a troubling precedent for retroactive liability generally.
