It's been a criminal offence in England and Wales (Scottish law is different to that of England and Wales but broadly similar in most cases) for generations to be found drunk on a highway, public place (can include a building) or licensed premises. The offence charged is 'found drunk', though often dealt with by way of caution or fixed penalty once sober, but the key thing for the police to take action is for the person to be incapable of looking after him/herself, hence the offence is usually called 'drunk and incapable'.
Police officers are often reluctant to arrest because there are many, many cases down the years of drunks dying in police cells either because they choked on vomit or had an underlying injury, often to the head, which was not apparent. Hospitals invariably won't take in people who are 'legless' so the unsatisfactory situation, for everyone, of the police cell being used as a refuge still obtains in many cases.
Typically, the police officer's evidence was, "I saw the defendant lying on the pavement. His eyes were glazed and his speech was slurred. His breath smelt strongly of intoxicating liquor. He was unable to stand without assistance."
If some-one was left lying on a pavement in such a condition and harm befell him, the police would be severely criticised and an officer could even find himself subject of a criminal offence. That's the dilemma that drunks can pose to law enforcement authorities.
There are other offences that people can commit whilst drunk in public such as behaving in a disorderly manner.
It's in no-one's interest for any person to be on an aircraft whilst drunk because such people's responses to the condition are so unpredictable, and can potentially involve danger to themselves and others, especially in an emergency situation.