The reference given in the Wikipedia article for the 1918 date of the song "Knees up, Mother Brown" is to a 1941 fiction, Random Harvest, by James Hilton (see pp. 208 and 211). I see no reason to suppose it was not simply a matter of narrative convenience for Hilton to date the song to 1918 in his novel. Similarly, the claim that the song originated in Britain on Armistice day appears fictional:
... The crowd were still singing "Knees Up, Mother Brown" in the bars below. It sounded new to him, both words and tune, and he wondered if it were something else he had forgotten. He did not know that no one anywhere had heard it before — that in some curious telepathic way it sprang up all over London on Armistice Night, in countless squares and streets and pubs; the living improvisation of a race to whom victory had come, not with the trumpet notes of a Siegfried, but as a common earth touch — a warm bawdy link with the mobs of the past, the other victorious Englands of Dickens, Shakespeare, Chaucer.
op. cit., p. 211.
OED attests the phrase in print from 1939 in Weston & Lee, Knees up Mother Brown!, a musical score printed by London and Sydney publishing houses.
However, the earliest use I could find was in The Illustrated Buffalo Express (Buffalo, New York) 06 Aug 1922, p. 9 (p. 5 as numbered in context; paywalled):
"KNEES UP, MOTHER BROWN!"
Sixteen English bathing beauties, the famous Tiller girls, entertain Madge Merritt of Ziegfeld fame at Brighton Beach, N.Y. Madge leads the file. — Cop't, Kadel & Herbert.
That 1922 reference to "English bathing beauties", along with the London and Sydney locations of the original publishing houses for the 1939 printing, does suggest that the song appeared prior to 1922 in England. A search of UK papers in The British Newspaper Archive, however, turned up no uses of the phrase or variants (for example, Brown/Browne) prior to 1927.
The phrase "knees up" is, of course, also suggestive — of ribaldry. OED, in an entry not "fully updated" since it was first published in 1976, sublimates any bawdy connotations with their "occasionally in extended uses" catchphrase, although the elliptical use of the shortened "knees-up" is remarked:
knees up, Mother Brown n.
A light-hearted popular song beginning thus; a popular dance in which the knees are vigorously raised to the accompaniment of the song. So elliptical, as knees-up n. spec. a lively party or gathering.