The Department for Transport and Security are part of the problem, certainly in the UK. Fortunately, they have little to do with an an aircraft in flight.
They write legislation (specifically chapter 10 of the SCD) which governs in flight security measures, it builds on EU legislation with more stringent measures. They also own the Transport Threats Team which is the POC between carrier and government during any in flight threats in UK airspace. I wouldn't describe that as "having little to do with an aircraft in flight".
Just as a minor point of order, they're called the Department for Transport by the way. No such thing as the Department for Transport and Security.
Besides, this happened on an inbound flight to the UK so local law takes precedent.
The Air Navigation Order is extant here, as it is on all UK registered aircraft regarding drunkenness on board, and that doesn't matter where the aircraft is flying to or from.