My only advice is that parents will be parents. It's their house, and at the end of the day if you're living in it you don't really have a leg to stand on - and they will exploit that loophole all they can because they're scared of anything happening to you. You can't just call it a "cheeky pint", though, as if that excuses anything; if they're so convinced that a pint is that bad, you might as well be talking about a "cheeky joint". You're complaining they can't see it from your point of view, but you can't seem to see it from theirs either! Telling them "it takes me 10 drinks to get smashed" is unlikely to make them see your point of view, rather it'll make them think "the next time he wants to get drunk, he's gonna fill himself with a gallon of beer", which they clearly don't want. You should be reassuring them you don't want to get drunk, you just want to socialise. And if you do want to get drunk, well... I dunno what to suggest. If they don't want you getting drunk and then returning to their house in that condition, you can't do much about it.
I think tact and discussion are the answers, and from this post you seem capable of neither. Don't just continue going out for your "cheeky" acts of insubordination, because that'll just make them even more protective of you - sit them down and discuss it calmly with them, reassure them that you're sensible and safe, and you might reach some sort of compromise.