I use the Catford -City Thameslink line but get off at Crofton Park in the evening, getting a bus the few stops if it is late at night. The trains on that line go from Victoria after 2143 at night which usually suits me but is something to bear ni mind. The London Bridge - Honor Oak Park line is generally very busy but there is safety in numbers - lots of people alight at HOP.
Most of the streets are residential and I generally feel quite safe, although it is wise to stay alert as is the case anywhere in London (difficult after a few drinks I know)

Good luck