I expect this actually to be less severe than some expect; if SWR do what Southern did, literally every manager or head office staff that can walk and read correctly will be trained as contingency.
I'm not sure how long that will be tenable though. The contingency staff have their own jobs to do and they're often going to be office staff probably used to doing a 9 to 5. Agreeing to do an operational role for a day or two ever few weeks is one thing but doing it for weeks on end? That's a very different proposition and I wonder quite how many of them will be willing to do that...