LONDON —A convicted fraudster with a violent criminal record, Tommy Robinson is well known as the leader of the English far right, inspiring rallies of mostly white, mostly male followers shouting soccer-style chants against Islam and immigration.

It has been jarring then, for many Britons to see Robinson — whose real name is Stephen Yaxley-Lennon — become the latest cause championed by the richest person on the planet, Elon Musk.

“Free Tommy Robinson!” the billionaire entrepreneur…