500' clear could be horizontal, think of it as a bubble. This DOES NOT apply for t/o and ldg. He may have started his approach.
Has anyone considered that if he was diverting and had been handed over by heathrow, he would have to retune his radio and be heads in for a few moments. How long between handover and impact? I stated earlier, when all is going to plan it is high workload in the lanes, something unexpected is tough to deal with, even in good weather. Battersea is not the easiest to spot at night if you are not familiar, lookout could be concentrated down and not ahead. So there are a few scenarios, all speculation.
God bless those who died, only the pilot himself will really know what happened, we can only guess.