As I understood it at the time, the principal reason (among several) that BA withdrew from Harare was it was proving impossible for them to transfer their ticket receipt funds out of the country, due to various government restrictions. No matter what the demand might be, or that London is the No 1 European destination from the country, if you can't get any of the revenue generated, what's the point of BA (or anyone else) doing the route ?
There is, of course, still a daily BA flight to Harare, the Comair franchise flies in from Johannesburg in full BA livery and BA flight numbers.
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