This is an interesting one. From my own 2 cents, I believe the normal/permitted reason to deny someone alcohol is if they appear to be drunk; to stop them getting worse. And you had ~4 hours left to sober up. I'm assuming you weren't slurring your words - I know a few people who'd be pretty much passed out on two drinks. Neither drinks much, both are short/slim.
I don't know for sure if you can make it through customs drunk - the level of drunkenness would have an impact I'm sure. I would love to have seen the FA's reaction if she'd told me that on either leg though given as my current status legally guarantes me entry into both the UK and Canada. They might throw me in jail if I can't stand up - but they can't deny me entry.
I believe customs might be allowed to temporarily refuse you entry and place you in protective custody until sober... as to if they then let you in or deport you (at AC's expense) I'll bow to the better judgement of others.
Or at least, what passes for free legal advice here on FT
And now, popcorn time.