London is one of the most important cities on the planet and the main driver for the UK▸ economy, you can't and shouldn't prevent a city from growing, it's a healthy sign but the infrastructure needs to keep up.
Is this not more or less a zero-sum game? Does 'not investing massive amounts of money in expanding public transport' equate to 'prevent[ing] a city from growing'?
Cities like Bristol have been extremely successful in reinventing themselves and are now booming - it's for others to follow their example and catch up, not the leader to slow down. We all benefit from a strong London.
There are no other cities like Bristol, and Bristol has not reinvented itself - it has been slowly growing and adapting (not always in good ways) for the last thousand years. That aside, the currently funded transport schemes there look pretty parsimonious and unambitious compared with what's being spent elsewhere.
Don't we also benefit from a strong Bristol, Leeds, Nottingham, Liverpool..?