For all those bemoaning the ground staff not being more proactive in preventing boarding of passengers who are showing signs of having had too many drinks, please consider that most Handling Agents are independent of the airline and therefore they do not have the complete authority to offload an airlines passenger without the airlines permission. If the airline have a Station Manager/Duty Manager the situation should be referred to them, if not, the Captain of the aircraft has the final say.
Sadly one crews drunk passenger and 'do not board them', is most crews 'they don't look too bad, will probably go to sleep once airborne'. After awhile the ground staff don't bother referring anything but the very worst cases as they pretty much know that the crew will accept the passenger.
If Jet2 are being consistent in refusing drunk/inebriated passengers from travelling then that is a big positive and the Handling Agents are clear they will be supported by the airline when the inevitable complaints come in weeks later.