I thought Labor and Corbyn were against Brexit? I think I read somewhere that he is in favor of giving the Falklands back to Argentina, so, I don't know.
Labour was split Remain/Leave, just like the Conservatives were split during the Referendum, more than once the Referendum was referred to as the Cameron/Osbourne vs Johnson/Gove Conservative Leadership Campaign Open To
All Voters Of All Party Allegiances And None.
Corbyn, himself, was nominally Remain, "7 out of 10 swayed towards Europe", but was criticised for not actually putting his weight into convincing people of that. Meanwhile, May was Remain-but-mostly-absent.
Anyway, Labour now has the problem of having to garner support from its old/potential supporters who were Leavers (who may have voted UKIP, and are now tempted to vote for President May because they like the cut of her gibb)
and old/potential supporters who are still Remain-inclined, and would probably wander in the direction of the LibDems if that gave the message (that voting Labour wouldn't, whatever else they think about Corbyn) that No Exit is still their preference.
The Tories have a degree of Remain haemorhraging to deal with, but are simultaneously likely being infused by UKIP voters, if that party doesn't stop its own bleedout, like it didn't for the Local Elections.