a route to LCY using F70
As a regular LCY user I can assure you that operations continue straightforwardly in Wet-Wet-Wet conditions; although associated low cloud can be more of an issue. I suspect the poor performance of the F70 was more the point, although they have been used regularly into there by a couple of operators, including Air France, over time.
The limiting factor in LCY departures is a bridge over the Thames east of the airport when on easterlies and its impact on engine-out performance. Regular users of the airport may feel they have not seen such a bridge. That is true, it has never been built - but it is planned, theoretically could be built one day, and thus has this theoretical impact on performance limits.
Back at St Helena, I'm starting to get the feeling that Comair have just got cold feet about the financials of the operation.