But there IS an Avios redeposit fee. Golds used to have this waived, but no longer. Only GGLs can avoid this fee [I think?]. Or Gold Priority Award bookings. It applies online as well as on the phone [the phone attracts an additional handling fee, except for Golds].

http://www.britishairways.com/en-us/...d-service-fees

That you could not cancel online perhaps indicates that you are now very close to departure. I often find that leaving things to the last minute (and, of course, you cannot cancel within 24 hours of departure) can make the online cancellation options problematic. Or it could just be a temporary glitch.