Why should it not be a Mayday call?? The aircraft lost 50% of its power and I was taught that situation = Mayday. On top of which the aircraft was UNABLE TO MAINTAIN HEIGHT and wasn't requesting descent, but telling ATC they were descending regardless, and that is 100% a Mayday.
As for my Spanish comrades, probably more a lack of experience with handling emergencies than anything else. UK ATC was one of the first to get a full and compulsary emergency training course going and now its a yearly session, however I know many others do it much different.