Why is there an indefinite article before "London"? [duplicate]

This construction gives a sense of renewal of acquaintance or a rediscovery. Note that you can also employ the definite article.

Example

*I returned to Anthea's house after many years to find an Anthea who no longer laughed and sang. She was now a sad reflection of the Anthea I used to know.*

You can think of it as meaning "a version of".

*I returned to Anthea's house after many years to find a version of Anthea who no longer laughed and sang. She was now a sad reflection of the version of Anthea I used to know.*

There is a sense that Anthea, or London, could be in many different states. You have returned to discover which state exists in real life.

I hope that helps.