Mary Berry (in-universe)

Mary Berry was a celebrity chef whom the Twelfth Doctor thought resembled his first incarnation; (PROSE: Twice Upon a Time) the Twelfth Doctor even referred to his earlier incarnation as Mary Berry, (TV: Twice Upon a Time) but the reference was lost on the First Doctor as he had yet to meet her, though eventually his third incarnation would spend a summer with her on a narrowboat in the Cotswolds. (PROSE: Twice Upon a Time)

Mary Berry attended the Twelfth Doctor's Christmas dinner party for Sprout Boy when he was feeling left out. She shared a Christmas cracker with John Luther. (TV: Sprout Boy meets a Galaxy of Stars)

Behind the scenes
Mary Berry portrayed herself in Looking for Pudsey.

Information from invalid sources
Mary Berry assisted Eddie Redmayne when he contacted the Twelfth Doctor in the TARDIS. She acted as his personal cook. (NOTVALID: Looking for Pudsey)