Search london history from Roman times to modern day
A bit out of date, but this interactive map of step free London is very good
I visited Southgate station in July 2019. Not much to say, there is a long elevator to exit or enter the platforms.
All the London railway stations :
Southgate station
Southgate station elevator
Underground lines : Piccadilly
Accessibility friendly : Southgate is not accessible to wheelchair users due to stairs or escalator between the platforms and street level.
Escalator access between street and platform.