//if the Conservative party is broken up , or was no more, who would be the main opposition party when Labour win the next G/E? //
Oh. Sorry, I didn't realise that's what you were getting at.
For the next election? Probably not much. There'd be huge disagreement about what that opposition should be - whether it should be centrists like En Marche (which people like Cable obviously want) or "proper" conservatives (which people like Jacob Rees Mogg would probably want). An opposition which was actually divided into different parties would almost certainly guarantee a Labour win. We're in the realm of hypotheticals here as the Conservative party is not going to collapse, but if it did, my money would be on the centrists winning out maybe after the current election cycle, and a Blair-like government after that.