Especially if a busy airport like LGW has some issue affecting the landing rate, you will get a lot of diversions fast and those diversion airports and their ATC will become saturated equally fast as well. There is no doubt that ATC in the south of the UK is absolutely top notch, but even they are human beings and a lot of diversions do require a lot of extra work.
Considering the airline in question has something like 65 to 70 aircraft based in LGW, even a short problem will invariably involve quite a few aircraft. If it was not anything you can plan for, they probably did not take extra fuel to account for something like that.
According to the rules there is basically two levels of low fuel warning from an aircraft to ATC: after having committed to land at a certain airport you may end up with less than final reserve fuel in case of any change to routing or clearance, it is "low fuel", that is not an emergency and the aircraft in question will not get preferential treatment. Well, it might, but there is automatism there. The next level is, if it is certain that you will land with less than final reserve fuel, it will go directly to "MAYDAY Fuel". The difference between low fuel and mayday fuel is surprisingly small.
Standard fuel planning does not account for any holding at the destination, or delay vectors while ATC sorts out a diversion and so on. It is departure to destination including approach and missed approach, diversion to alternate, and then 30 minutes of holding (final reserve). On top there is of course fuel for taxi out and contingency, but that can be as low as 5 minutes and can be used at any time after off blocks for any reason, so it might not be available at the destination.
I had it myself, waiting 15+ minutes until ATC had sorted out a diversion, it can happen. And the fuel used during that time does not have to be planned for, so it can dip into final reserve very fast indeed.