The flight takes 25+ minutes. Dumping that alleged quantity would take 25+ minutes, all at least 6000' above ground level. The whole of the south of England wasn't going to go up if someone started a car engine! Nobody would even sniff it- it will vaporise. Overweight landings are 'an emergency only procedure' and not to be used in the course of normal operations. Fuel dumping is an 'emergency only procedure' as well- there are subsequent ramifications to the fuel gallery and engineering procedures.
I still find the alleged explanation garbled. It may well have taken place, but the hints at confusion and lack of knowledge don't make any sense. It is standard procedure to confirm aircraft weights and all it takes is an eye to confirm fuel on board for the flight- all this is automatically in the FMS system so full that knowledge is confirmed by a computerised loadsheet. Nobody was 'caught out'. I suspect it was late reassignment of a heavily fuelled aeroplane to maintenance 25 minutes away. Cardiff is not that long for a max landing weight 747. But before too many expressions of astonishment are made, I would like some positive confirmation of exactly what happened.