Just checked for my trip to Canada in May (BRU - LHR - YVR) and I'm able to select exit seats for the flight BRU - LHR (already selected on, but could change). Same for the way back (YYC - LHR - BRU). No exit seats on both LH flights as I'm in WTP
For LH, the BA site is quite clear: " Exit row seats are not available to Silver or Bronze Members on long haul flights until 24 hours before departure. "