Being a member of Balpa I fully understand & support the views being expressed here.
However there is a ticking time bomb at LHR with the coverted 14% of prime time slots held by bmi. Lufthansa has a 30% stake in bmi, and just like BA had with the GB slots/airline at LGW, has first call on the purchase of bmi. This option matures soon. Likewise SAS owns 20% of bmi and is hoping to sell its stake soon. All three of these airlines are part of the Star Alliance, an arch rival of BA.
Just to add a little more interest to the Airline ownership situation at LHR, it is heavily rumoured that Singapore Airlines is seeking to sell its 50% stake in Virgin. Virgin is of course non aligned but SIA is part of the Star Alliance.
All the above is I believe the prime reason for PL.