The problem with this thread (apart from the emotion) is that the evidence cited appears to be anecdotal or personal. There is no doubt that some baggage contents are stolen, regardless of the airport or airline: Travel enough and it will happen to you eventually.
What we are missing are facts. Facts would enable us to ascertain if BA has a greater problem than other carriers at LHR, and also if LHR has a greater problem than other airports: Facts would also demonstrate whether BA & LHR have improved in this respect over the past decade or two.
I anyone has facts, please share them.