Here’s another argument that won’t cut it:

“It was a non-binding referendum so it doesn’t count.”

The deal was ask the people and the Government was elected on both a) giving that referendum and b) abiding by its result.

Most MPs thought it would never pass.

You are correct, legally, they could ignore it. But democracy is a not a one-step Game Theory scenario; ignoring it will end this Government forever. Consider again the lawyer with the idiot for a client. She can give the best advice possible, but if the client insists they will try to sue a baseless complaint, the lawyer has to follow their instruction.

How about if Trump said after the next US election, “Yeah you voted me out overwhelmingly, but I actually know better than you so we’ll ignore that.”?

We cannot get any sort of trade deal until the withdrawal deal is passed.
I believe the government is trying its hardest to deliver on its instruction, but yes they know it will damage the economy. Half the voters are just not listening.

Hope this helps you understand their impossible dilemma.

CL