I'm not comparing them to the UK, but the most populous countries in the world. Among the top 10 most populous countries in the world, only 1 (the US) features on the list you shared. I'm making the point that population mass doesn't dictate a country's political weight in today's era. This includes the UK, which as you pointed out has a relatively small population but is globally still influential.
The reason countries with more people appear powerful today is historical and twofold: a large labour force (as opposed to skilled labour pool, or advanced technological industries) during agricultural/industrial/manufacturing times for increased economic output, and more critical mass for premodern imperialist/colonialist territorial turf wars. Those reasons no longer apply today, but the legacy remains – many of the most populous or colonialist countries back then are first world countries today (including the UK).
Again, the most populous countries in the world (even China and India) don't generally rank very highly relative to their population size, and most are 3rd world countries as well.
Moreover, you've only listed all the countries tied for #1, conveniently ignoring all the other countries within the top #5-10 I mentioned which have smaller populations than just London: Singapore, the Nordic countries, European countries. It's literally there on the list. These countries tend to have the highest skilled labour forces.
I stopped replying the other poster because my mind was just rotting reading their replies. Yes, there's no doubt a large labour force (skilled or unskilled) has its benefits, but both your beliefs that the government is trying to expand the population for political power is simply ridiculous. In fact, the other poster implied the government wanted to increase population density, which is an even more disastrous recipe in modern times.
Sorry if this is rude, but I'm one of those foreigners (ironically "imported" for my economics expertise) the other poster mentioned, and if both your beliefs are common, my mind is simply blown at the level of education in the UK.