But there is a difference between a population who speaks English and a population that speaks English as a first language.
There are five countries that have a larger English speaking population than the UK: India, Nigeria, Pakistan, Indonesia, and the Philippines. Yet, a lot of the cultural output of those nations isn't in English because those countries have first languages which take priority.
So the commenters of a post may be fairly diverse, but the post is likely going to come from the perspective of someone who speaks English as a first language. In that case, English speakers are likely going to know more about what is happening in the UK over India because British news is likely written in English while Indian news is going to be written in a wider variety of languages.
I've mentioned in other cases that a lot of cultural post content is generally created mostly by people's first languages. Local news is likely going to be written in the local language. Discussions on media are likely going to be in the language the media was produced in unless it is a major national hit.
I've mentioned that five more countries have English-speaking populations larger than the UK, but you're likely going to see a lot more post content on the UK over Indonesia because local Indonesian media likely isn't producing the quantity of English coverage as the UK. And I don't expect that to change, as such a change would mean killing local languages.
You're focusing on the nationality of the readers. Most of the people arguing that the USA is the largest English-speaking country is focusing on those providing the written content.