This may be a dumb question, but will BA let me print a boarding pass at an airport which I'm not flying out of on BA?

Next month I'm flying from Canada to LHR on Air Canada, and then connecting to a BA ticket; I'd like to get my BA boarding pass printed before I arrive at LHR in order to save time on the connection. (Yes, I know that bad things will happen if my incoming flight is late -- I have 3 hours to make the connection and the AC incoming is usually on time so I'm not too worried about this possibility.) There is a BA check-in counter at my originating airport, but I don't know if they can check me in for *any* BA flights or only if I'm leaving that airport on BA.