All this discussion about schedule time ignores that, on my experience from the last two trips out of LHR, the average time from doors closed to wheels up was 75 minutes. The last two from LCY, about 5-10 mins.
Living close to LCY, the days of morning riverside fog that disrupt ops are about 5 to 8 per year. More often than not cleared by 0930. In over 100 flights through there it has never delayed me.