I think it needs to be done over the phone thats if it is possible as i dont think you can do open jaw tickets all that easily via ba.com
sounds like your in an ideal position to try and do a bit of a tp run while your over there and bump up your status!!
something to keep in mind however, the Canada flights depart LHR and the Caribbean islands return into LGW, so depending on how your getting to LHR for your outbound it could land you with a few difficulties
cs