We should be honest here. I understand completely why you'd want drinking legal on the tube. The reason is what we here call 'pre-drinking'. You're going out somewhere where you'll be drinking, but it's going to cost a lot of money, or you're going to a party and want to be drunk when you get there.
So, better to get drunk as fast as possible, as cheaply as possible. Because it's illegal to drink while driving or taking public transit here, we do/did our predrinking at home or at a friend's house.
But if it was legal to drink on public transit, we'd have done it there also, to extend the predrinking as long as possible. [Except for the diff that here, everyone had a motorcycle or car from age 14 on, and public transit was for losers.
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Problem is that if you admit to this truth, you'd be admitting that Boris has a point, because let's face it, some people aren't that good at their predrinking, and they'd become loud drunk nuisances on the tube or bus.
That's about the long and short of it, as I see it.