I would put it to you that the precedent has already been set. There is no robust system in place to impeach or remove a Prime Minister who is proven to have lied. The system relies on honour and is broken when the person installed is incapable of acting honourably.On the face of it, I would agree with you. But, this is now going far beyond party politics and into the way the United Kingdom is run as a whole. We currently have a government who appear to be more than happy to have even the most basic standards of public life thrown out the window.
Yes, it's easy to say "play the long game" and we'll be fine at the next election. But I would also say consider the even longer game. If we allow the government of today to set a precedent that this sort of behaviour can go without punishment, then where does it end? How do we guarantee that come the next election that someone else comes into power and doesn't do the same thing, or worse? We are actively demonstrating that the current checks and balances we have in place for public servants can be ignored simply by saying "sorry"...repeatedly. That should never be the case, when previous governments have seen such actions to be nothing less than a resigning matter.
The more the consequences for failing to uphold these standards are watered down or outright ignored, the more dangerous this could become. The UK has sat back on the view that it's the "Mother of Parliament" for a long time, and watched as other countries descend toward becoming more autocratic with the view that "it could never happen here". We're in danger of sleepwalking into that exact situation in the UK, which may sound overdramatic to some, but it's a very slippery slope!
The man wont resign. His party wont remove him, let alone vote with the opposition in a confidence motion.
Anybody can challenge him but he doesn't have to answer, and even if he does and is later proven to have lied, he can't be called a liar.
It's a broken system. The time for protecting precedent has been and gone. The opportunity for a new progressive government to cite how broken the system has been proved to be as rationale for wide-scale political reform is clear, whether anyone has the balls to do such a thing this decade is another argument altogether.