Considering that all the other formerly British Commonwealth countries (Australia, Canada, India, Ireland, New Zealand, Pakistan, and South Africa) all use metric, with whom will Britain trade. Rolls-Royce and Bentley are German owned, Jaguar and Land Rover are Indian owned.
Metric system notwithstanding, the huge majority of data about materials properties, mechanics, chemistry, and other stuff engineers rely on was formulated in the Imperial system. Conversion is a herculean task that will take a century or more.
Have you ever heard of the United States?
He’s not doing away with the metric system, just allowing it in retail transactions which the EU forbade. If the local fruit vendor wants to sell bananas by the pound, he may.