I think BMW, Mercedes, Porsche and Volvo, can go back to Europe and are fully compliant with European standards even the more stringent British ones.

Other European makes as well as Honda I think are nearly there compliant so few insignificant changes, Toyota, Lexus, Mitsubishi and Nissan may need a few more minor mods, but I don't think they are a major problem.

US cars need quite a few changes to make them road legal for Europe

Indian/Chinese cars like Great Wall, Mahindra, Maruti etc not even worth thinking about, a) you wouldn't want to drive them here, and b) it would be cheaper and easier to buy a BMW than modify them.