In the Xenophobe's guide to the English, page 54, under the heading Sense of Humour, the authors, Antony Miall and David Milsted, state that:
English humour is as much about recognition as it is about their ability to laugh at themselves, for example: 'I thought my mother was a rotten cook, but at least her gravy used to move about a bit.'
I am not certain I get the joke. Does 'move about' have two 'opposite' meanings on which the joke plays?
Googling the joke, I got a reference to another book, Smoking in Bed: Conversations with Bruce Robinson, which reads:
Growing up through the radio age you had Ray Galton and Alan Simpson's brilliant dialogues for Tony Hancock. Lines like 'I thought my mother was a rotten cook, but at least her gravy used to move about' are indelible in my mind.
Best Answer
The style of joke is formulaic, relying on comparison; you thought some other thing was bad , but at least it isn't [bad in the way this thing is]. By comparison, other bad things seem less bad.
The quoted joke has been removed from context, but the formula tells us that the speaker's mother is being contrasted with some other, previously referred to person, whose cooking is execrable.
The quote in question, although no reference has carried forward into your snippet, comes from a Radio, and later TV, sit-com called 'Hancock's Half Hour', in this instance Series 5, Episode 14.
The surrounding lines are
Part of the joke here also relies on the audience's foreknowledge of the characters and actors who play them. Hattie is played by Hattie Jacques, a a comedy actress, large by standards of the day.
So there isn't any particular joke about the gravy, other than speaker's mother makes bad gravy, but at least it's runny. 'Moves about' just meaning 'is not completely solid, as Hattie's becomes on the plate.
I might take issue with the idea that this is 'about their ability to laugh at themselves,' Tony is clearly laughing both at his mother and the unfortunate Hattie.