There is history, and there is now.
On the historic side, BA CC were the best mix of professionalism and service for me back in the 90s when I did two to four l/h per month. Whilst the asian carriers tried harder, the BA crew were just more professional. (I was BA gold and SQ gold).
However, time moves on, and all airlines are only as good as the CC were on your/my last flight. As the consultants point out, every "moment of truth" counts. So the lgw crew might be universally brilliant, but if you experience an lhr group on a bad day, then that is your memory of the airline.