Boris doesn;t strike me as the jail type. I don;t think we'll see him in a ditch any time soon either.
In any case, the parliamentary situation is different. Doesn't parliament pick the PM, that's why he's the PRime. So can't parliament unelect him? If they don;t, then in effect they agree he should remain PM even though he violated their own law. In effect, they annulled their own law.
This is similar to what happens in America when we go to war. The Congress is supposed to declare war, not the president. That's in keeping with our founders desire to limit power of one man. However, over the years, Congress has ignored that rule and given more power to the president to get us involved in conflicts without their "official" OK. However, they could stop the war by not funding it. Troops have to be fed, paid, and armed. So if the Congress continues to fund the war, they in effect have declared war constitutionally. At least that's how the Supreme Court has ruled in the past. So if Parliament similarly does nothing to the PM, they de facto agreed with his act even if it was illegal de jure.