In the UK London is currently the largest city by a large degree at around 8.5 million with the next biggest city being Birmingham at just over a million people, with the third largest city being Leeds at 750,000 people and 589,000 and 552,000 to Glasgow and Sheffield respectively. How would it be possible for the population to be spread around to other cities such as Manchester, Bristol, Glasgow, Liverpool, Cardiff for example preferably to over 600,000 each?
This could be done in several ways such as expanding the borders of the cities, encourage immigration to cities other than the capital and increased investment in the other cities.
Other than that I am not sure how this can come about and I will appreciate any feedback and answers.