We have done that. After changing at Clapham Jcn onto a packed Southern train (is that not tautology?) for Gatwick, with cases, we saw some spare seats, which other standing pax were ignoring. After collapsing, after 5 mins, I noticed the First signs. We had assumed that the standing pax were piling out at E Croydon (they usually do) and thus were not bothering with a seat. Southern is a bit like being on the Tube, frankly.
It is only like being on the tube on services within London. Once you're outside, it is like being anywhere else. Given the first class seats are identical to their standard counterparts in the front and rear coaches of their Electrostars, you'd be forgiven for thinking it was standard. Especially on certain types of unit where there is no separating door between first and standard.