This is the Future that corresponds most directly to the path the Naturalist was trying to follow, altering himself to alter his Destiny. As the name implies, Altered Future.

Part of what's going on is that this Future is only possible because of your experience in Irem, and that you alter yourself in direct response to your possible Destiny. That process of altering your future in response to your future, that feedback cycle is itself a Future consisting in aggregate of the Destinies that you perceive and your responses to them.

This is not a Future of London. This is a Future for you, if you choose it. London is left well behind.

In addition a large part of the experience of this Future is that your alterations are comprehensive and include your sense organs, and probably also your brain and consequently your mind. It's not London that's different, it's your perception of London that has become so radically alien.