I too can't believe either thal London City, Northolt, etc are happily turning their back on any executive jet operations, apart from those who want to depart in the overnight hours. But if you can afford your own jet you can normally afford a hotel, and avoid flying through the night.
Regarding the London Underground, I understand that the unions do not accept any planned overtime at all, and have not done for some time, trying to keep their numbers up (their income being dependent on the number of menbers, of course, not what they may earn) and thus, as there are no additional trained and qualified personnel, it becomes problematic to schedule any extra hours of operation.