Just tried to book LCY/JFK MFU over the phone as I can't get it to work on-line.
Told by agent and duty manager that MFUs are not possible LCY/JFK, although straight miles bookings are.
Apparently, this is no longer an option online as well. I successfully a MFU online back in January but now it is not possible. Although straight miles bookings are indeed possible. Perhaps someone at BA noticed this loophole and fixed it?