An actor represents Brangwyn as charming and bubbly as he discusses working from photo reference: "Took it from a photograph. People didn't talk about using photographs. 'Not done! Cheating!' Rubbish! Saved time and money, in addition to a sketchbook. Tremendously useful."
Wikipedia on Frank Brangwyn
Book: Frank Brangwyn 1867-1956
Thanks, James Warhola
Corrections: The initial post was incorrect in assuming that the on-screen persona is the real Brangwyn. According to the producers, "The Brangwyn in the film is an actor...The great actor Sir Donald Sinden, who played Brangwyn's father in the film, suggested his friend and colleague, Rowland Davis, to play the part as the physical likeness was so extraordinary." The post has been revised to incorporate this correction.