If the AAIB couldn't manage to determine the amount of fuel on board before the flight in a 5 month investigation, it would seem to be a reasonable defence that the pilot might have difficulty too.
Reasonable that the pilot may not have been able determine the amount remaining, yes.
Reasonable as an excuse for running out, no.
If you know you burn 100L p/h and you're planning a 2 hour trip, then you should uplift 200L for the trip, plus 75L for the hold, plus enough to divert, plus 5-10%.
If you can't fit that much in the tanks, you know you're full
If you can, then worst case is there's a bit more left when you've finished flying the trip than there was before... (Ignoring W&B for a moment)