Kinda missing the point there. Lets say that all factors considered, repairing the aircraft costs 1 million more than pulling one out of storage and retuning to service
It's not even that complicated.
AFAIK, BA aren't bringing a 747 back from the desert to replace G-BNLL (though they have done so in the past), they are simply deferring the retirement of another aircraft that is currently in service but was due to be parked (albeit at the cost of an additional check).