I may be misunderstanding something here but wouldn't it be a relatively easy business case if BA were confident they could sell some 1 hour farewell flights at say £500 a pop? Particularly if these were 1-way flights (to CWL for example) then part of the business case is already made as a lot of the costs of this last flight will already have been accounted as part of the 744 fleet withdrawal. There are about 10-12 744s still at LHR that will need to be flown out, these are the ones.

At £500 I do think they would sell out in a matter of hours.. To maintain social distancing. lets say 150 seats on offer at £500 each would generate £75,000 income.....for a flight that has to happen anyway! Up-thread I read BA has had some negative publicity recently re taking delivery of 787 / 350 whilst making thousands redundant (I haven't read this myself)....what great PR BA could create out of this though....maybe a few seats on each flight being auctioned for charity etc etc.

Notwithstanding what others have pointed out, if there is a sudden increase in Covid-19 outbreaks again then of course this would influence things and plans would need to change. But heavily promoted as taking all social distancing into account with a strict limit on # of tickets / charity auction of some seats...plus these flights need to happen anyway (this part I think is really, really key to promote as it mitigates the Greta crowd) I really can't see a negative PR angle at all.