I do believe the stories of check-in staff allowing intoxicated Pax to board and cabin crew for doing their best to ignore them or, supplying more alcohol in the hope that they will go to sleep.
I have seen this from Luton a couple of times in 2001 when I was commuting LTN~EDI every week for six months on EZY. The worst was a football crowd that had attended a game in Europe and were transiting at LTN. They had obviously been drinking after the match and at transit and were now boarding the last of the day to home. I was fortunate, in that they were very noisy but not offensive. Also, I guessed correctly that they would use the front steps and so used the rear set to be far enough away from them. Others were not so lucky on what was always a busy flight.
I also believe the story because I have seen gate staff ignore many aspects of pax behaviour because, if they intervene they have a problem, if they let it go, they do not. If they let it go and, later, someone in authority pulls them up - it was only one amongst many that they let through. i.e. the odds are in their favour.
I am sure that CC on the last sector of the day are making a balance between delaying the flight to offload and the chance that the person will fall asleep. Again, the odds are in their favour. I do not blame the CC and FC for this, it is human nature and if the pax are not a threat to good order? If they are only a nuisance? Think of the example given about train service from Leeds.
That is real life in a commercial operation and they will be considering their airline getting a reputation for occasionally letting on some noisy pax or getting a reputation for always offloading and then having to fight a different
PR battle.
Lastly, I believe it because the pax that is mildly to averagely intoxicated KNOWS that he has to walk carefully and not say anything out of place whilst he boards. If he is completely drunk, then he will lose control and be offloaded but he no longer cares. So, actually, it is the ones that are NOT fully drunk that are the bigger problem as they still have some brain cells functioning.
But, as to the question do FC + CC connive with drinking pax in the hope that they will remain quiet for the 60/60/120 min sector? Yes, of course and we see reports of it in here a couple of times a year.
As to the cultural/national origins of such people - I have no doubt that you can see this same behaviour from ANY local/regional UK airport. Consequently, if you say they are one cultural/national group over another, it is no slur on them - as it would be the same for any airport. Those rising to defend their local cultural/national group are missing the point. The point is - what to do about airport and airline staff allowing intoxicated pax (of any nation) on to an aircraft.