I was wondering the same myself only the other day.
I'm sure I recall landing into Jomo, definitely on a BA flight, in the morning on one occasion. I know I departed LHR, T4, on Saturday 3rd November 2007, arriving into Nairobi the following morning. I don't fly VS and it was a non-stop flight, so there must have been an overnight BA service into Jomo at some point.
Would be interested if someone could shed some light on this!