Before this thread runs to 100's of pages of speculation, we need more FACTS. The crew may well have said "MAYDAY we do not have enough fuel to divert" but it is certainly not in the BA book of words and I cannot see why it would have been said and thus I believe the reporting is (as always?) inaccurate.
MAYDAY is only required (as with Ryanair) when you EXPECT TO LAND with less than reserves and has nothing to do with diversion fuel. In this situation I guess a PAN would have been an option, or some other form of 'advice' to ATC that they were now 'committed' to the field.
As to why they APPEAR to have 'committed' to an airfield that did not APPEAR to meet the EUOPS rules for such we will have to wait and see.