Currently we have no system for creating client sub-categories. I'm not sure if this has come up before but we'll certainly monitor any ongoing interest.
In the mean time you could possibly use the account reference box for grouping.
In this example the group name would be "LONDON" and you could assign any clients in that group with the prefix. Then on the client list screen you could key this tag into the advanced search to bring up all members of the group.
You could also potentially add a custom side link to easily isolate that group with one click.