I don’t know about the UK but I don’t think it’s illegal to board a plane drunk in the USA (unless you’re part of the crew). You can sit in the terminal bars and drink for hours upon hours and get on board.
That said, I’ve been on enough flights with belligerant drunks to suggest Airlines should stop drunks from boarding planes ... breathalyzers?
It wouldn’t be fair to those who can hold their drink and not be morons ... but it should be considered. Flying is a priveledge, not a right. Maybe they could have special booze flights for those who like to fly loaded. I personally would patronize an airline that profiled and that breathalyzed passengers.
Of course, there is is still the possibility of getting on board with a sober jerk ... but that’s the same no matter where you are.
Now that you mention it, there used to be an airlane that advertised “champagne flights” with the following slogan:
“Western Airlines, the only way to fly.” This was said by a cartoon big bird riding/reclining on the tail of the airplane. It was shown on television repeatedly and a very popular ad. Times have changed.