Ian, how have you learned of the missing son, and is your informant/source clear on the dates and details?
The Tony whose dates you have given seems to have been born Tony Sidwell, to parents Stanley Sidwell and Lillian née Jennings, and become a “Brett” on Lillian’s subsequent marriage to Wilfred Brett in 1933. His birth and death occurred in Lancashire. As Annette says, there’s no obvious link to your London Bretts, unless you can shed any further light.