This is oldest movie she has a major part in. Looks very different as a young mill girl in Yorkshire. On youtube.
https://www.imdb.com/title/tt0041203/
https://www.agefotostock.com/age/en/Stock-Images/Rights-Managed/MDO-1401347
Not quite the oldest - she has a notable role alongside Dirk Bogarde in Quartet, an anthology film, released a year earlier in 1948.
I haven't seen A Boy, a Girl and a Bike - thanks for the tip!
More pictures of her career in movies and the Avengers, plus politics. Is Brian Jones picking his nose?
https://www.theguardian.com/film/gallery/2020/apr/06/honor-blackman-a-life-in-pictures
Patrick Macnee and Honor Blackman - Kinky Boots (TOTP Video) https://www.youtube.com/watch?v=j0mTRiZGvvw