In reply to BnB:
With the UK's exit already in motion, the only ways for parliament to block no deal are to repeal article 50 (which I really can't see happening), or to agree a plan amongst themselves then get the EU to agree to it; the UK parliament can't unilaterally force the EU to agree to anything other than by repealing article 50.
It's still quite possible that the UK parliament could still pass the existing deal the EU has already agreed to (May's deal); it isn't completely impossible they could find common ground that the EU could agree to, but they've shown no sign of doing so so far, and they're almost out of time.
If Boris or another hardline brexiteer takes the leadership, unless the UK has already committed to a specific course of action, they can just run down the clock and it isn't obvious how parliament can stop them.