Changes

The train route between London and Brighton is very rarely checked. It is even more rare that it is checked after 9pm at night. There are 2 companies which do this route. First Capital Connect(FCC) Thameslink and Southern. Southern is checked more, FCC Thameslink very rarely. Theoretically you don't need to have a ticket if you travel after 9pm any day.

This info is out of date, in last year they've been checking ticket a lot more on both Southern and First Capital connectThameslink...edit 2018 : Been taking Thameslink in the last year as well and it still is pretty relaxed with controllers. i.e. controllers don`t live on trains, so they get on and off between them on Thameslink. Basically it's like public transport in cities, you might find them but generally you will be okay, especially in the evening.

You can take a train from Preston Park(no barriers, one station after Brighton station, walkable from the center, takes 25/30 minutes) that stops at either Elephant and Castle(no barriers, south London) or Blackfriars(barriers open in the evening, a bit more central than Elephant, can change here and take a train to elephant if your initial train did't go there directly) or Farringdon(this HAS barriers but from here you can change between national rail and underground and try to get to another station without barriers in other parts of London. One such station is Finsbury Park. There is a lot stuff to know about free transport in London, best bet is to ask your local squatter.)