Robbie, who played Sharon Tate in the film, stepped in to pick up the trophy and read Pitt's speech, explaining that the actor was unable to attend the ceremony due to a "family matter".
Opening with a cracking Brexit gag – "Hey Britain, heard you just became single - welcome to the club" – Pitt's speech continued with the star suggesting he was going to name his award "Harry", because he was looking forward to "really excited about bringing [it] back to the States."
Pitt beat The Irishman's Al Pacino and Joe Pesci, plus Anthony Hopkins (The Two Popes) and Tom Hanks (A Beautiful Day In The Neighborhood) to win in his category.
Robbie was twice-nominated at the BAFTA, for turns in Once Upon... and Bombshell, but lost out to Marriage Story's Laura Dern in the Supporting Actress category.
Tarantino's script for Once Upon.. was also up for best Original Screenplay, but that accolade went to Han Jin Won and Bong Joon-Ho's Parasite.