Having studied the IAA report, my conclusion is that it is the planned destination-to-alternate profile which should be reviewed.
If your planning assumed a 150 mile diversion, climbing to FL310 and descending for a VFR approach - but you then found yourself stuck down at FL100 and being vectored around the ying-yang by ATCOs close to meltdown, then you're on a hiding to nothing.
Many years ago, 'no delay expected' at London Airport meant that no delays in excess of 20 min could be expected - a somewhat different thing. Some airlines didn't know this and reportedly found themselves on fumes on a few occasions....
So I think I would take a leaf out of Dan Winterland's book, if I was unfortunate enough to be flying aluminium people-tubes, particularly in Espanish airspace in iffy Wx. Add allowances for at least 50 nm destination-to-alternate distance, assume worst FL, ISA+15 and engine anti-icing required - the heck with gauge errors and nearest kilogram calcs, there'd be a prudent fuel quantity at the alternate to avoid the need to declare an emergency!
About a million years ago, following the problems faced by a colleague conducting a diversion, we were ordered to declare a Mayday in any similar event.... Though not on CAT aircraft, I hasten to add. But after my wingman had a severe engine surge on take-off, returned to the aerodrome and blacked the RW after destroying the barrier, I had to divert. One would feel rather daft declaring a Mayday when well over max. landing weight, so I didn't! There then followed a few glorious minutes of aerobatics, before I landed my trusty Hunter F(GA) Mk9 at the alternate. The moral of which is that there might be SOPs, whether you're flying a Hunter or an airliner, but the commander must make the ultimate decision based upon experience and sound airmanship.