It pulled maybe 100,000. Not three million.
https://fullfact.org/culture-and-society/3-million-unite-the-kingdom/
Second, yes it would have. It was an anti-migrant march at its heart. Kirk was just the gravy.
https://www.npr.org/2025/09/13/g-s1-88663/london-protest-by-far-right-activist
Third, it was planned as early as October 2024.