I'm trying to identify a novel I have read in the 90s, but possibly date back to the 70s, 60s, or even earlier.
The Earth is entirely covered with ice and snow, and people live in an underground city somewhere in North America. The story starts with a group of scientists and scholars being banished from the city to the surface. I don't remember why, but it was political, they broke a taboo or something (and it may have been for trying to communicate with the outside) Before leaving, they are allowed to take tools and food with them. They decide to trek across the frozen Atlantic ocean, toward Europe. They encounter several difficulties: I remember at some point they fight wolves, and later primitive tribesmen trying to steal their stuff.
In the end, they meet a British patrol. Then some members of the patrol admit they were sent to intercept and kill them before they reach under-London. I think the final twist was something like this: there are several cities of survivors across the globe, each thinking they're the last remnant of civilization, and the leaders want their respective population to keep thinking that because it makes them easier to control.