Corbyn is definitely a Eurosceptic, but both he and McDonnell rule out a 'no-deal' Brexit. So the spectrum of the Labour Party dealing with Brexit goes from Customs Union, with a fudge of keeping close alignment with EU over worker protections, environment, etc.etc all the way through to outright Remain, probably via a 'People's Vote.' This is a long way from the Tory Party, where they have a not insignificant amount of head-bangers who actively want no-deal, all the way to May whose deal still falls short of Labour's starting position.
Corbyn and McDonnell are very close to getting into number 10 and attempting to not only reverse Tory austerity, but move the whole Overton Window to the left and swing us away from Thatcherism/Neo-Liberalism/whatever you want to call it. IMO, they are not going to trash this opportunity over Brexit. The way the party has handled it so far has been to let the Tories drown in their own mess, and Brexit is a Tory mess.