BOAC started using 707's in 1960.
British Overseas Airways Corporation (BOAC) was the official long haul airline of Britain from 1946 to 1974 when it merged with BEA (British European Airways) and became British Airways. I don't suppose I have to remind you wthat at the time Kenya was a British colony and would have had BOAC service to it?
True, but not on all routes. According to your link, by 1961 they were only using 707's to fly to the following:
Zurich, London, Tehran, New York, Los Angeles, Washington San Francisco, Tokyo, and Hong Kong.
I don't see Nairobi on that list.
In point of fact, the idea that 707's would be flying to such a backwater place only 2 years after being introducted is just plain laughable.
Face the facts: if you wanted to fly to Nairobi in 1961, you were flying by prop. plane. Period.