I see a couple of differences between the UK approach and Canada, or at least Ontario (it differs from province to province).

The legal alcohol level is point zero eight here as well--but below that, you are not considered safe to drive, you are just probably not committing a felony. If you test between .05 and .08 police here can issue you a 24-hour licence suspension on the spot, you park your car and find another way home unless you have a sober passenger (and you are a dope for not letting them drive in the first place.) Not a bad deal really, you don't even get a fine.

As often seen on TV, police can and often do ask you to perform some coordination tests (in addition to the breathalyser) to see if, in their judgment, your coordination is impaired enough to make you an unsafe driver.

We had a case last spring where a fellow who had a medical marijuana permit was stopped while smoking a joint. He was charged with impaired driving based on swerving all over the road. He was a retired lawyer as it happened, and defended himself. Believe it or not, he successfully argued that he was swerving not because he was stoned, but because of his multiple sclerosis...a known condition to the licence bureau.   

Technically, you have a point about alcohol levels, maybe. There is no single ultimately-accurate test that will determine your fitness to drive if you are in a borderline condition. I seriously don't think pupil-dilation is one either--it's just a simple physical response test which needs to be taken in context with other signs and tests. Doesn't really tell enough on its own.

Don't know about UK, but the typical impaired driving charge here is issued to someone who is way over the limit, two to three times the legal. Someone with multiple convictions who has regained their licence may have one of those breathalyser devices attached to the ignition of their vehicle, and have to blow into it and pass before the car will start. Most failures to pass these checks do in fact occur the morning after.

I understand what you are getting at and in one sense I would agree that you have a point--but ultimately I think the response would be that we all have an obligation to be as responsible as possible in regards to avoiding behaviour that would make us less than the best drivers we can be. That would however be draconian and unenforceable in law, so ultimately there are going to be arbitrary measures, which in good enforcement are combined with coordination tests and reasonable judgment on the part of the police, so they don't waste their time booking someone who has had two beers while a truly loaded guy is lurching along the same road five minutes later.