DeathSlam

Thanks for the pointer. It's an interesting read.
I think the answer is "It depends" and "Are you being a d*ck or obviously p*ssed".
Clearly not illegal by UK law at time of press.
Being allowed to consume alcohol but not being allowed to be drunk is a bit like being allowed to eat food but not be overweight. If you want a definition of drunk see above.
The intention here is that we don't have a safety problem with intoxicated passengers. This happens frequently and an airline that ignores it is not meeting their duty of care. Setting down a standard for this is going to be difficult. A complete moron who downs 2L of duty free vodka in 30 mins and causes a diversion due to their behaviour is clearly committing a criminal offensive. On the other side of the scale a frequent traveller who has 50 or 100mL of their favourite beverage that isn't available for BoB so should not be dissuaded. A set of three people who crack open a bottle of rum and go through it with free cokes and ice I can see as being either side of the line. I have certainly drunk more than that in CC monitored beverage distribution and only once in 30 travelling years on BA been told 'last one' but never actually denied.