Hi, Does anyone know what platform the southern train goes into at London Bridge and also what platform they normally depart from?.
I ask because I one of those who now needs to go to London Bridge due to the ELL closure!.
I am trying to find the quickest route to the underground from the platforms.

It varies. Platforms 8, 10, 13, and 14 are particularly used for Forest Hill services, with others also used.

I think you just exit from whichever platform you arrive at and head for the escalator to the underground, which is in the main part of the station.

Trains starting from Charing Cross will usually come into London Bridge on platform 5.

Trains to Charing Cross & Bedford usually stop at platform 6.
Make sure you get the right train!

Quickest route.

Out of underground. Into smaller overland entrance on the London Dungeon side of the station. Up onto platform 5 then up onto the platform access bridge. Check the destination board to see where the next train the Forest Hill is leaving from then run.

Figure out if it's a 4 or 8 coach train and chose carriage accordingly.

This route also has the added advantage of not having to run the gauntlet of all the sadly highly expensive and seriously fattening delicious yummies being offered for sale in the undercroft leading to the main set of escalators.