Indeed, but that won't stop them.
If Brexit is not delivered, both the Tories and Labour will suffer at the ballot box, but the Tories will be decimated and the net result will be a Labour government (possibly propped up by the SNP). Maybe 5 years of Corbyn/Sturgeon is just the sort of reality check the country needs.
Mind you, the irony meter would go off the scale if the EU commission blocked a Corbyn government budget like it has done with Italy.