Moving a little off topic (away from Jet2 specifically) why, apart from as a revenue earner, does alcohol need to be sold on aircraft. If it's wine, and therefore I assume beer and other beverages, it never tastes the same / as good as it does on terra firma. Unless you're a raving alcoholic going without for the duration of a flight ought not be a problem.
Moreover, surely if the problem of pre-flight drinking is going to be seriously addressed, then making it clear in the T&Cs that intoxicated passengers (above specified blood alcohol limits) will be denied boarding, and having a breathalyser at the gate that a member of trained security staff can insist suspects use before boarding.
Where there is a will, there is a way.
The downsides is of course reduction of revenue from on board sales, and reduction in revenue for airport bars and restaurants - and therefore also for airport operators. But when it comes down to it, surely the comfort and safety, particularly safety ought really be "paramount" as it is always claimed to be in any corporate statement.