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?
Pan Am acquired at least five 707 in 1958 and there may have been more.(I got tired of reading the small print on the Pan Am historical site)