I'm not in the least surprised by the success of EK. STN is not just a London airport, it's the regional airport for a large and prosperous population. My late sister was a great world traveller and would have welcomed not having to get herself to LHR.
Emirates flies A380s out of LGW as well as LHR but like BHX, some are High Y and No F.
Given STN launched with the standard 77W with First, it’s a proper standout from almost evey other flight at the airport. As for yields, volume gives no clue.
But the catchment area is comparably much smaller than LHR/LGW, draw a circle with STN at the centre and you have a lot of green fields, fewer if you do so at LGW, and much fewer for LHR.