Stand 18
The airport wish to make stand 17 larger to accommodate a larger 'Dash/Embraer E75' sized aircraft, which in turn, means stand 18 will become smaller.
The only advantage I can see in the airport doing this is to allow St17 to be used as a 'Night stopping' stand. Although, this now means BA will be restricted primarily to Stand 41/Gate 141 which is quite possibly the worst stand/gate combination in the entire airport for both staff and passengers alike.