My dad was a tunnels engineer and worked on Crossrail feasibility studies at several points in his career across decades.
London is is many ways one of the less impressive subway systems simply because much of it is so old, with small trains running in Victorian era tunnels. Not as bad as the Glasgow one, which feels like travelling on a 2/3 scale model of a subway with alarmingly narrow platforms.
It is however a point of contention within the UK that London public transport is better than public transport in almost every other city, due to being properly nationalized.