If you see incidents of anti-social behaviour or begging on trains please report it.

Southern Railway has a page on their website about reporting vandalism, fare evasion and anti-social behaviour.
https://www.southernrailway.com/help-and-support/journey-problems/eyewitness
Govia Thameslink has a similar page on their website. https://www.thameslinkrailway.com/help-and-support/journey-problems/eyewitness
Both trains have a regular service announcement explaining how to report incidents and posters displaying the telephone numbers to contact.

Guidance given by the British Transport Police is to report any issues or incidents on the train or at a railway station as soon as possible on 0800 405040 or text 61016. If someone is in danger then call 999 immediately. Those numbers can also be used to seek crime prevention advice or to make the Police aware of issues affecting your local station or your journey on the rail network.

The train companies and British Transport Police rely on people reporting incidents in order to tackle issues and enable officers to target the areas where there is the most need. The most effective action to reduce instances of anti-social behaviour or begging is to report any incident as soon as possible so that whoever witnesses it can give as much information to the train company or Police as possible. The more people report incidents and give descriptions, times, trains etc., the more likely the Police are to catch the perpetrators.