The relevant process is
https://en.wikipedia.org/wiki/Preferential_attachmentIt's been "preferential" to "attach" to London since Roman times, ie., if your ROI is proportional to your output, then london has been "the place to spend" for 2000 years.
The industrial revolution "levelled up" parts of the north which had advantageous geography and resources, but that was a period of "growth redistribution" which acted against the tidal forces of preferential attachment.
Absent something outside the basic logic of ROI-tomorrow prop-to ROI-today (eg., ports, coal, etc.), that is what dominates.
Gov. policy here is pretty hopeless; you can make marginal improvements, but it is strictly, and quite literally, a waste of money.
Both for each £1 of gov spending, and each £1 of private spending, more impact on your goals are achieved in London (et al.).
Eg., one has to argue £10mil on some local transport for a <few hundred to use a week is worthwhile, whilst £10mil in london would enable 1000s/week.
It isnt, and in any case, makes little difference to the overall picture. "Levelling up" then means that the gov redistributes money from economically productive areas, to unproductive ones, to make marginal improvements.
There's a limited amount any gov can actually do that; too much and there's nothing left to redistribute.