From the article lionheart linked (my bold):
"Andy Wigmore, Leave.EU’s communications director, revealed to this paper that Farage was a close friend of both Bannon and Mercer. He said that the Leave campaign was a “petri dish” for the Trump campaign. 'We shared a lot of information because what they were trying to do and what we were trying to do had massive parallels.'
"Wigmore also said that Mercer had been 'happy to help' and Cambridge Analytica had given its services to the campaign for free. It was the general secretary of Ukip, a British lawyer called Matthew Richardson, who effected Leave.eu’s introduction to Cambridge Analytica, Wigmore said. 'We had a guy called Matthew Richardson who’d known Nigel for a long time and he’s always looked after the Mercers. The Mercers had said that here’s this company that we think might be useful.'
"He said that Mercer, Farage and co had all met at a conference in Washington. “The best dinner we ever went to. Around that table were all the rejects of the political world. And the rejects of the political world are now effectively in the White House. It’s extraordinary. Jeff Sessions. [Former national security adviser Michael] Flynn, the whole lot of them. They were all there.'
"When the Observer revealed Mercer’s 'help' in February, a 'gift' of services, it triggered two investigations. One by the Information Commissioner’s Office about possible illegal use of data. And another by the Electoral Commission. Cambridge Analytica is a US company and Mercer is a US citizen and British law, designed to protect its electoral system from outside influence, expressly forbids donations from foreign – or impermissible – donors. The commission is also looking into the 'help' that Gunster gave the campaign. It was not declared in Leave.EU’s spending returns and if donated, it would also be impermissible. Gavin Millar QC, an expert in electoral law, says it raises questions of the utmost importance about the influence of an American citizen in a UK election."