The Iranian response to the seizure of its tanker in the Strait of Gibraltar, by the seizure of a British Tanker in the Strait of Hormuz, was also illegal, though more understandable as a reaction. The implications for the global economy of the collapse of the crucial international law on passage through straits would be devastating.
It may seem improbable that the UK and or France would ever seek to close the Dover Strait, but in the current crazed climate it is no longer quite impossible to imagine the UK seeking to mess up access to Rotterdam and Hamburg. It is still easier to imagine them seeking to close the Dover Strait against the Russian Navy. Yet the essential freedom of navigation through the Kerch strait, respected by Russia which controls it, is necessary to the survival of Ukraine as a country. For Turkey to close the Bosphorus would be catastrophic and is a historically recurring possibility.