It's similar, but we have the deterrent of if you are over the limit, you will be penalised. You only have to look at the culture shift in attitudes to drink driving after this was introduced to see the positive effect it has had.
Utilitarianism doesn't work on libertarians, and a libertarian society wouldn't consider this a valid function of government. Laws in a libertarian society are about protecting the rights of citizens. To a libertarian, murder should be illegal because it violates your right to life, whether or not it deters murderers is irrelevant. In this case, drunk driving isn't an inherent rights violation so there's no justification to arrest people for it. However, if you accidentally hit someone with your car, and it can be proven that it wouldn't have happened "but for" your drinking (and thus your negligent disregard for another's rights), it's a more serious crime.
Unfortunately from past experiences I don't share the same faith as you in the humble bobby.
I don't really understand what your fear is with it though. Whether or not your drinking was a contributing factor isn't up to the police to decide, you wouldn't be arrested for drunk driving because there would be no crime for drunk driving. You'd be arrested for whatever traffic violation happened, whether that's driving recklessly, running someone over, speeding, or hitting another car, and the rest would happen in court. If you shoot someone the police don't arrest you and sentence you to life in prison, they arrest you because they have probable cause you've committed murder and then you'll be prosecuted in court. I don't see how this is any different.
....giving them the power. How do you prove I was driving unsafely? CCTV? Fair enough, what happens if I say there was a bee in the car?
They already have this power, police don't need ironclad proof to detain you, they need reasonable suspicion that you're involved in a crime to pull you over, and need probable cause to arrest you. An officer observing a car swerving all over the road is probable cause that a crime is being committed, because swerving all over the road
is a crime. As it is now, they pull you over for swerving, give you a breathalyzer, and then arrest you for drunk driving. Without a drunk driving law, you'd be pulled over for swerving, arrested for reckless driving, and the officer would likely note if you were drunk.
Then it goes to court, and the question is whether or not you were driving recklessly, not whether or not you were drunk. The prosecution would have to prove that you were driving recklessly, and you would try to either disprove that or create reasonable doubt that you were. You could try to say there was a bee in the car, but I doubt that would be accepted in court as a very thorough defense.
EDIT:
@FoolKiller summed up what I was trying to say much more succinctly.