12 penalty points ≠ ban

What I am saying is that you would be battling perception - it is widely accepted that the current limit is fair, I think people would view a total ban as unfair.

All of that aside it will have zero effect on the people who currently flout the limit.

Would reducing the drink drive limit to zero be the best use of time and resources in terms of accident prevention? How would it stop those who knowingly drink and drive now?

Why do people obey a law?

I think you are too pessimistic. People obey the law for complex reasons; fear of being caught is one of them. But with regard to drink driving it is not only that. The argument has been won. I am old enough to remember how resistant people were to changing their behaviour and all the bogus arguments they put forward (they were really quite similar to the ones people going to Forum drinks put forward actually). But over time the anti-drink driving argument won and now people accept it because they see it as valid and for their own good. In fact I think a lot of people, members of my own family for example (to dip in to anecdote as evidence) stay well below the limit and would not actually be outraged if the limit was reduced to zero.
You say people knowingly drink and drive and of course there is going to be small hard-core of selfish idiots who do that regularly but there is another larger group who do it occasionally and probably feel bad about it. I think it there is every chance that their behaviour will be affected for the better if the limit is lowered and what you might call the cultural pressure not to drink and drive is strengthened. Considerably more severe penalties for people who are well over the limit would also play a part. Bit by bit these changes can be made. They take time and effort. In the seventies and even in to the eighties there would have been people making the same arguments as you are now. Because of the efforts of politicians and police and campaigners in overcoming those arguments hundreds of people continued to live who would otherwise have died. I don't think we have reached a point where we can go no further and should accept that the number of drink related deaths on the roads is as low as it ever can be.