They will have banned the overtime to put as much financial pressure on staff as possible and stop them making extra money to part cover losses due to strike days.To run an extra service at the last minute as a Controller you'd ring the local crew manager and ask what spare train crew or cover crew you had, in this instance the only depots of use would be Brighton and Eastbourne, the answer straightaway would be NONE owing to the management banning any guards from working overtime.
Normally you'd have at least one spare crew at one of the 2 depots so you'd pinch a train from the sidings , at Eastbourne you would have something you could use off of a commuter train from London then just run an ad hoc service to get everyone where they wanted to be. I did it many times in the past, on such occasions as the Fat Boy Slim gig on the beach when they forgot to put any extra trains on and realised they had 250k punters wanting to get home.
Trouble is now Southern just don't care, they want to paint the narrative of Conductor sickness being the reason behind the cancellations when the reality is that they have never had enough crew for years and relied on overtime to patch it up and because they want to carry on the bullying and being vindictive they would rather cancel trains and leave people stranded so they can keep up their message of it being due to Conductors being sick.
They do this knowing full well how it will inconvenience their customers.
Southern care nothing for the traveling public.