Are the aircraft flying a 5% or greater approach? They would be, of course, because that's what it says the threshold is calculated on when they checked the appropriate documentation as they planned their flight, so why would they fly lower? They might hit something.
I've got no real skin in this game other than an interested observer but for those who have can you at least stop making statements that you have no idea of. London airport (not Heathrow)uses a 5.5% gradient. I doubt that 30 is predicated on one.
Only multi-engine, fixed-wing aircraft up to Airbus A318 size with special aircraft and aircrew certification to fly 5.5° approaches are allowed to conduct operations at London City Airport.[7][8]