If the antidemocrats think they have a case at law, they can apply for a judicial review. But they'd lose it and even they are admitting it.
Parliament is in utter deadlock. The people spoke in 2016. Parliament voted for Article 50 to give effect to the will of this majority. Boris's government is putting into effect Article 50. He has a huge amount of support
If Leave ever held the democratic high ground, Cummings and Johnson have surrendered it with this act of cowardice.