Why is 'Mr. Brexit' concentrating his tariffs on the UK?

Tariffs always cause the most damage to the country that imposes them.

That is one of the few propositions to which every economist of the Left and Right can assent. Sure, tariffs also do incidental harm to overseas companies, but their weight is mainly borne by domestic consumers. We can argue about exactly how heavy the burden is. The National Taxpayers Union Foundation says that President Trump’s import taxes cost the average American family $425 a year. J.P. Morgan puts the figure at $1,000. Trade Partnership Worldwide says $2,300.