I've found a bit more for you. lmperial Airways 1938. The first day is Southampton to Athens, and the second to Alexandria. "The flying-boat then proceeds to Cairo and flies over the Nile Valley Luxor and Wadi Halfa....On the morning of the third day from England, the first stage is to Khartoum....The trunk route southwards from Khartoum follows the White Nile to Malakal...and continues to Port Bell on the northern shore of Lake Victoria. It then turns eastward to Kisimu, which is the end of the third day's flight from England" It seems these flights were operated by Shorts Empire flying-boats. I can't find frequencies, but the route to India was five times a week, so perhaps similar to Khartoum. Of interest is the airfare. London-Luxor return was £ 90, inclusive of all meals and hotel accommodation. If I find anything more I'll post again.