You can increase frequency in London on many lines (e.g. the Northern) by resignalling, and TfL has been looking at doing so. But it ain’t cheap. See London Rec

You can increase frequency in London on many lines (e.g. the Northern) by resignalling, and TfL has been looking at doing so. But it ain’t cheap. See London Reconnections passim. But the bigger wins are more active travel (by providing segregated cycle routes) and moving more Tube journeys to the shoulder peak.