Just my thoughts but I feel you are looking at the wrong flight! the BA001 leaves when it does because it is the arrival time of the BA002 that matters! That is the direct flight that can really get the yields and it does indeed get everyone to LCY for 0700 and a full days work.
leaving an aircraft on stand at LCY is an expensive buisiness so it leaves pretty much as soon as feasible (bear in mind also the reporting times of the operating crew). Even if BA wanted to pay for a later flight doubt LCA would accept as it would leave just 3 stands for all the domestics and the Swiss C series.