I disagree with many things you said here.
"Leave the physical ability out of this, it is about psychological ability to be responsible."
Personally, my opinion is that being a soldier requires both physical ability and mental responsibility. So, determining an age range for military service requires taking both of these into account.
However, the drinking age does not require taking physical ability into account. That's why it makes sense to have an older minimum drinking age than the minimum military service age. That is also why it's OK to allow 60 year olds to drink but it's not OK to allow 60 year olds to join the military.
Out of curiosity, why do you feel it's necessary to leave physical ability out of the consideration for military service?
"Drinking age in Britain is 18. Is anyone going to argue that British 18 year olds are vastly different to American 18 year olds"
Many things differentiate British and American 18 year olds. We do drive a lot more in America, including our 18 year olds. That is one huge difference between American and British 18 year olds. I'm sure there are others too. If drinking is going to be legal, that's one reason why I support having a 21 year old (or older) drinking age.
One thing I've noticed is that binge drinking is very common in British society. The UK's overall rate of binge drinking is nearly double the USA's. The UK also reports nearly a 30% rate -- 3 times America's -- of high school aged students binge drinking 3 times or more in the past month. I would posit that has a lot to do with Britain's drinking culture and establishments revolving around both including and serving 18-21 year olds.
Browsing through the statistics, it looks like the UK has a major problem with alcoholism. Perhaps, Britain should also raise its drinking age to 21.