When the BBMF gets to be unsupportable
That would be when the devil calls in a heating engineer because it's getting a bit chilly. Aren't the BBMF kites the only ones with no planned OSD in the RAF Management Plan? The way we are going as a country and an air force, BBMF will be all we have left!!
Then again a Lanc fitted out with LGB or small diameter bomb might be quite useful in Afghanistan. Long endurance and large payload!!!