"Mind the gap" seems nowadays to be used at any London Underground station where there's anything that might just be considered a gap. (Of course now it's a pre-recorded message). In my childhood the announcement was always made live, but only at stations where they were gaps that might be dangerous. I associate it above all with the oldest section of the Bakerloo Line (Baker Street-Waterloo - hence the name "Bakerloo".
Incidentally, I've encountered the expression as a jocular and polite way of telling someone that the area around the waistline isn't covered by any clothing ...

(or should it be

?)
As for safety on the London Underground I can even remember the days when there were still a handful of pre-WWI trains in service with doors didn't shut automatically. I remember getting on a crowded train with my father when I was aged about five. The train pulled away from the station and I was right next to an open door. Within seconds the train was in the tunnel and it was a terrifying experience for me at that age. After what seemed an eternity someone noticed and slid that wretched door shut.