I think there could certainly be institutional deadlock and possibly competing legal challenges, though I'm not sure it would technically be a constitutional crisis. It will partly be events unfolding, a lack of obvious protocol about how to cope with a rejected deal. Politicians losing control to some extent (though they rather than us are still able to reset the rules of the game). However I think the interesting issue is whether this institutional deadlock triggers anything real, away from Westminster. There were claims that 'brexit betrayed' will bring people onto the streets, maybe even violence, whereas the dominant emotion at the moment seems to resignation and boredom.