To add to what the other commenter said - Bank station (the busiest on the London tube network) is getting rebuilt too, to add a "mezzanine" type area for the Northern line. Through-trains, all to the same destinations, are every 2 minutes and the platform is frequently closed due to overcrowding. To the point, in fact, where there's often a one-way system enforced in the station, or people are kept at street level.

Ultimately it's usage levels and capacity. UK stations don't have the capacity for the passengers, and often the capacity on the trains either.