The problem with anyone trying to divorce politics from ethics, epistemology or metaphysics is that your opponents are not playing by the same rules.
There is a popularly accepted belief that is sorely in need of debunking. A government is legitimate if it has been elected by the majority of the governed, and thus authorised to make decisions by, or rule the entire populace on behalf of, that majority. That British governments are elected by the majority of the governed, and are thus authorised by those majorities to make decisions or rule on their behalf. That therefore British governments are legitimate.