The minimum fuel on board
Trip
diversion
final reserve
taxi
5% contingency
The company, like most, take advantage of reducing the contingency down to 3% to save on fuel, this could of played a part if it was used.
Low level holding fuel is not added to the fuel calculations, hence why there was not enough fuel to do it. The plane has to land with no less than 30 minutes of holding fuel. The captain could of used his diversion fuel if he felt assured of a guaranteed landing at LGW. Obviously, this wasn't the case and the crew diverted.
The crew, now diverted, will make it a 3 sector day which will reduce the flight Duty Period, which in turn put them into discretion, if not already due to the late departure.