If you look at homicide rates, London's is ~half of e.g. New York's, but the countries are really, really different in a lot of ways. It would make more sense to compare the U.S. to the European Union than to the U.K. Compare the U.K. to Massachusetts or something. The U.K. doesn't have a long land border with a violent, half-ruled-by-gangs developing country, doesn't have large rural areas, etc.

The available evidence suggests that magically removing guns from the U.S. overnight would make a dent in homicide rates, but not by all that much (I would estimate ~20%, charitably). Americans murder much more than average not because they have access to guns, but access to guns does make them a little more effective at murdering.

An aside, but it's always interesting to me that people are specifically interested in instances where a lot of people die together. I mean, who cares?