If it were my itinerary I'd take one on the MIA-LHR leg because it's an overnight and morning in London is also (roughly) morning in Nairobi, so you're easing into Nairobi time. I'd try and stay awake on the flight to Nairobi so that I'd be tired enough to sleep on arrival, consistent with local time. If I woke up 3 hours after going to bed in Nairobi I might take another one if I had to be functioning at my job the next day- if I were on vacation I'd just read for awhile and hope to get back to sleep. This is similar to a trip I took to India in February although I stayed overnight in London both ways, which helped coniderably.

But after taking Ambien 2 nights in a row I'd stop and just try to let my body deal with it.