My thought is about why we might prohibit sales to drunk people. My guess (which was backed up by James Nicholls from Alcohol Research UK) is that drunk people pose more of a threat in terms of accidents and disorders, and that the marginal health costs are greater for each additional unit - think J-curve.
So it seems that the law is in place to stop people getting "too drunk". Now of course, someone could get "too drunk" by buying a plentiful supply of alcohol from the supermarket earlier in the day. No-one would be able to stop the individual from getting "too drunk" in this scenario.
However, the individual could not purchase the alcohol in the supermarket if they were drunk, so they have to decide beforehand that they want to get drunk. This makes me think that it is illegal to serve drunk people because they are presumed to be acting irrationally when drunk. We know that people do stupid things when they are drunk - count the number of stolen traffic cones in students' halls - but how do we know that their purchasing of more alcohol when drunk is irrational?
What makes me question this is that individuals often choose to go out when sober, often choosing which venue to go to and (perhaps crucially) how much they are willing to spend. Taking £30 to the pub, knowing full well that you will be drunk when you buy your last drink, suggests that buying alcohol isn't necessarily an irrational decision - you decided to spend that money when you were sober.
If what we're doing isn't irrational, and we can drink ourselves silly at home anyway, perhaps this law needs some justification.