Now this isn't to say I didn't drink under age, cause I did. Did my mother know? Yes. And she was somewhat irresponsible to allow me to continue to do so, and I was wholly irresponsible to do so. I did know parents that would buy their children alcohol and they were 100% irresponsible, however my mother did not ever buy me alcohol, which is somewhat responsible.
I'm also British. I know what the law is here. But picking a particular age (in this case 16) seems silly when there are plenty of countries where that would be perfectly acceptable. Anyway, that's not really important.
Teenagers are going to drink whilst they're underaged. It's something that's always going to happen and it's not really something that you can stop from happening. You can try to make it harder for them to obtain alcohol (bars and shops have gotten a lot better at not selling alcohol to underaged people; I was going to pubs when I was 15 but I wouldn't get away with that now), but the system is never going to be perfect. They're going to get alcohol from somewhere and thinking otherwise is naive. I know this from experience; either someone else might buy it for them or someone amongst them will look old enough to get it themselves.
You say that parents buying alcohol for their children is irresponsible, but I don't think that's always the case. If the children are already showing an interest in it and if they're already likely to get alcohol anyway then is it really irresponsible for the parents to provide a safe environment for their children to do something that they were going to do anyway? Parents buying alcohol for their children allows the parents to keep an eye on their children whilst they drink, which is ultimately a lot safer than them drinking unsupervised on a park or whatever. It also allows the parents to have more control over how much gets drunk, what gets drunk, and how quickly it gets drunk, which is again safer. Whether or not it's irresponsible isn't so black and white once you stop assuming that the law will be upheld 100% wrt. teenagers getting alcohol (because it won't).