The film is set in a city after a huge catastrophe. It is about a small group who are living a pretty brutal life in the ruins. In one scene a stranger ventures into a subway station and is attacked by feral children. At the end they escape through a window back into the 19th century. I think the film is British and flips between the devastated future and the past. I think it was made in the 70s or 80s. Anyone know the name?