Britain with a much larger population is going to need a lot more skyscrapers to house those people. Otherwise, it will have to build over a bunch more nature and replace it with a concrete jungle.
It really doesn't. Apartment blocks between 3 and 5 storeys are very space efficient housing solutions, if such buildings became the predominant form of housing in a Britain with 130 million people, you could house the entire population on a smaller amount of urban land than the UK uses currently.
Abortion is never made leagl i the whole uk. As i understand it thats about 8m exstra people. And if the pill is never made legal who knows how many exstra that is.
Well, generally, countries with abortion restrictions have experienced savage demographic declines. The path of restricting women's rights and access to healthcare seems to slow population growth in the long run, even if the first generation under such a system grows faster.
In general I would say faster growth in the 20th Century could reasonably count on a mix of:
1) A Britain that is able to accommodate Ireland within itself, rather than losing most of the island as per OTL.
2) A more economically interventionist state between 1918 and 1939, meaning the British economy doesn't essentially stagnate between the wars, likely requires the Liberals to hold power for most of this period.
3) A better prepared Britain that is able to defeat Germany faster and with lower costs in WW2, or else WW2 is completely avoided.
4) Britain follows a pro-industry development path post 1945, rather than following OTL's pro-finance development path, which have led to the collapse of British industry and the inflation of a housing bubble that makes starting a family very expensive indeed.
5) A Britain whose political dynamics favour some combination of welcoming immigration or limiting emigration (for example, a tense relationship with the USA that means that a favoured destination for British emigration is no longer welcoming).
So basically, a Britain where the economy was so robust that people could afford to have families earlier and thus tend to have more children, combined with being a beacon for immigrants and a place would-be emigrants want to stay.
I'm not sure all of these things would get Britain above the Japanese population though. Maybe if we cheated a bit and count Dominions in federation with England, Scotland, Wales and Ireland? Indeed, if we just added Canada, Australia, New Zealand and South Africa to the UK population we'd be there without a need for economic and demographic PoDs.
fasquardon