The flights to South Africa give an E value of around 60-70%, as it's about 12 hours each way plus approximately 12 hours on the ground, which would mean that about two-thirds of the time away is spent in the air.

If I presume that the aircraft operating BA 326 LHR-CDG returns the following morning as BA 303 - the result is an E value of an awful 22%! I have based my opinion purely on the quantitative data, overlooking all the qualitative factors that would obviously justify this value.

Edit: The aircraft that operates BA's last flight of the day to MAN, BA 1404, returning the following morning as BA 1385, yields an incredibly low E value of just 18%.