SAllyann
I would appear to have mis-interpreted your posting, it wasn't clear to me your intention, for which I apologise.
20Driver
I’d suspect BA does not keep back up crew in Delhi so right away the time frame for a new crew has a minimum dimension.
BA frequently has no backup flight crew at LHR, and certainly none in Delhi! Even at home base they rely on people answering a phone when not on call and then 'force' them to work on their days off. You don't become one of the worlds most profitable airlines by having spare flight crew.