I had a similar experience a couple of years ago when we lived in Feltham (W. London).
I used to live in a block of flats that had it's own underground garages for the residents. I was getting in to my car one day when one of the neighbours walked (well, staggered really) past me absolutely reeking of booze and got in to his own car.
I then proceeded to watch as he spent the best part of 5 minutes attempting to reverse out of his parking space and collided with no less than 2 different parked vehicles, as well as managing to make contact with the concrete dividing walls at least 3 times.
Now I really couldn't give a flying f*ck if somebody decides to drive without tax, etc and I am most certainly not a grass, however it was obvious to the registered blind that this guy wasn't just a little bit over the top but that he was absolutely rotten drunk.
I decided to phone the police on my mobile and follow the guy so I could give them a running commentary of where he was heading so he could be apprehended.
I explained to the police operator what I had seen and also that I was following the individual and that his driving still hadn't improved in the slightest from the car park incident.
The operators first response was to point out to me that making a phone call while driving was illegal
I pointed out that I was using a hands free kit, and asked if she would concentrate on the more important matter in hand and get this drunken twat off the road before he either hurt or killed someone.
Long story short......
I was told to stop following the guy as it wasn't my responsibility to get involved, and that now I had furnished them with his registration number they would deal with it.
After telling the police operator that she was a stupid tart (mainly because she was more concerned about someone making a phone call while driving rather than the pisshead behind the wheel that couldn't even walk properly due to his intoxicated state), I followed the guy to his destination anyway and sure enough, we didn't encounter one single police patrol along the way (which meant he got away with it).