LONDON—Two crypto billionaires made the biggest political donations in British history to Nigel Farage’s Reform UK, in a bid to revitalize the anti-immigration party that has been beset by funding scandals and slowing momentum in the polls.

British-born businessmen Ben Delo and Christopher Harborne said they each gave £36 million, equivalent to $48.7 million, to Reform UK to help professionalize the upstart party ahead of the next general election expected by 2029.