It can't really be separated from LHR's overall performance since BA is over half of flights, just under half of passengers (and would be well over if you added the 3 other IAG airlines). But yes there has been a concerted Heathrow wide effort to improve performance since it feeds a lot into the logic that LHR is competing with AMS, FRA, CDG in terms of being a European hub with global reach. There is a liaison committee which sits between the airlines and the airport, and a lot of their work is trying to iron out the various small things that can get in the way of on time performance. However it has to be said the biggest factor is simply the weather. BA and HAL can get LHR to work well on 97% of capacity, but once there is fog or wind or thunderstorms then something has to give.