I think the current leader gets to stand by default (ISTR some hoo-hah around this point when Corbyn was challenged by Owen thingy last year). Here we go:
https://en.wikipedia.org/wiki/Labour...ship_challenge
'On 12 July the National Executive Committee ruled that as the incumbent, Corbyn would automatically be included on the ballot by an 18–14 vote.[11] The party's lawyers, GRM Law, as well as James Goudie, had argued the party's constitution required Corbyn to secure nominations,[61][62] but conflicting legal advice obtained by the Labour Party leadership and UNITE from Doughty Street Chambers and Michael Mansfield, respectively, argued Corbyn should not need to obtain MP/MEP support to be placed on the ballot of a leadership election, as the party rules only mentioned the need for challengers to receive nominations, and did not explicitly specify the same requirement for the incumbent.[61][63][64]'