First class on operators around London and the southeast is really just a way of guaranteeing a seat (you can apply for a refund of the fare difference if you have to stand), since in many cases it's the same seating arrangement as you get in standard class.
Many commuter operators around London have now abolished first class - particularly post-Covid - on the basis that there isn't sufficient demand to warrant the resultant loss of standard class capacity. Southern are a bit of an outlier both in retaining first class, and in setting the first class fares they price at only 1.5× the equivalent standard fare - whereas most other operators have increased the multiple to 1.6, 1.7 or even 1.8× the standard class fare.
On regional and longer distance services, there is much more of a case for it as you will typically get 1+2 seating, and may get some catering - depending on the service in question and the time of day/day of the week.