I don’t often have to use Southern Trains (South West Trains cover most of my regular patch) but when I have, I’ve just turned up and shoved the bike on.
They don’t seem to have many dedicated bike areas on their trains (and the ones they do have are p155 poor in terms of location and capacity) so sometimes I just have to put it in the wheelchair bays and hang around (so I can shift it if the space is needed by a wheelchair user). This would mean I get to stand with my bike in the aisle/end of carriage/door areas. Not comfy, but hardly catastrophic.
I’ve even gotten on to trains from Clapham Junction around 5.30pm on a weekday with a (non-folding) bike although I did have to wait for a train with few enough people in it for me+bike to fit.