EK is slot limited at LHR so every flight needs to be on an A380 as EK needs every F seat it can get from LHR. STN actually has a quite wealthy catchment area including tech/biopharma corporates around Cambridge etc, as well as being the easiest to access airport for a lot of the finance industry based in East London, which is probably what warrants it. In addition 6 F seats probably works better than 8 in old 777 F (I think similar applies to likes of ZRH, GVA, NCE etc. where the population in the catchment area is not massive but includes some pockets of very high wealth that is willing to pay a premium). It's not a co-incidence STN is the only airport to get new F twice a day.