In OTL there was a *very* close race for a seat in Parliament between the two not long after World War I. Should Mosely beat Chamberlain, how does the trajectory of each man's career change? Will Mosely still go fascist? What becomes of Chamberlain?
I think chamberlain for a long time was in the shadow of his brother Austen, the peace advocate.
Mosley might have ended up party leader, and fought under a democratic banner.
Mosley was too much of a maverick ever to be Labour Leader whether first elected under that party's banner in 1924 or 1926. As for Neville Chamberlain, I assume he could win a safe Conservative seat in a by-election before too long.