The 90-year-old, who joined the King's official birthday celebrations following the death of his wife, Katharine, Duchess of Kent, last year, stood on the far right of the balcony on his own.
Her father, Richard, and mother, Katharine, were devotees of the religious charity the School of Economic Science (SES), likened to a cult by its critics.
Katharine Birbalsingh said she was due to attend the event on Thursday to discuss the motion: 'This house believes that being British is a birthright, not a choice.'
Today is the Duke of Kent's 90th birthday - an occasion that will likely be marked by sadness following the death of his wife, Katharine, aged 92 last month.
Katharine Birbalsingh, known as Britain's strictest headteacher, lashed out at Bridget Phillipson for imposing her will on schools without any experience on the ground.
The 1,200-square-foot home sits on a half-acre wooded plot with stunning views of a canyon. It has also seen numerous upgrades since it was Hepburn's stomping grounds.
On Tuesday September 16, Katharine - who preferred to be called Mrs Kent - was laid to rest at Westminster Cathedral, surrounded by Royal Family members.
Sophie, 60, gathered at Westminster Cathedral with other senior royals to bid a final farewell to Katharine, who was one of the most popular royals of her generation.