What does "you'll know me again" mean in British English?

I'm reading The Lives of Christopher Chant by Diana Wynne Jones, and someone says to a stranger who is helping him, "You'll know me again, young lady." What does that mean?

I think it must be a British English idiom, because Google says that "you'll know me again" is also used in Salome and the Head and Bleak House. But I don't know those works well enough to triangulate a meaning.

Solution 1:

In the Bleak House extract, Mrs. Snagsby gives Weevle 'a searching glance'. Presumably the young lady in your book had been looking closely at the man she was helping.

I don't remember hearing anyone say it in real life, but the expression means 'You are staring so hard at me that you are sure to recognise me if we meet again.'