Brexit. “Everyone knows who won, but not everyone knows how,” says Benedict Cumberbatch’s character, Dominic Cummings, in the opening moments of this film, which has already ignited controversy in Britain. Cummings led the campaign in 2016 to have the United Kingdom leave the European Union, though the actual exit is still very much in the air as the deadline approaches. (Think of making a film about special counsel Robert Mueller’s investigation before it’s over.) Written by James Graham, whose excellent play about Rupert Murdoch, Ink, is to come to Broadway this year, it’s an incisive, if dispiriting, look at what now goes into selling voters — including those who usually sit out elections — starting with their Facebook pages. 9 p.m. Saturday, HBO.