Michael Moore went on and on about how Canadians in big cities don't lock their doors, showing in one of his movies how you can just walk up to any house and open the front door and walk in. Compared to places in the United States, he was flabbergasted that this was true in Toronto, which is a very large fairly cosmopolitan city.
In Some parts of London it isn't safe to leave a door open even if you are at home.