For quite a while, brasil was the metropolis and Portugal the colony (as the Royal family moved to Rio).

Not really (source: am a Portuguese currently living in Portugal).

Kids here can immitate a Brasilian accent, and so can many if not most adults, because maybe 4 decades ago Brasilian soap operas became all the rage in Portuguese TV, but they don't go around normally speaking with a Brasilian accent.

Then again I can immitate a number of US regional accents (well enough to fool Brits) and a number of British regional accents (well enough to fool Americans) when speaking English, but that's not at all the same as generally speaking with that accent (though, having lived over a decade in London, my English language accent tends towards RP English, also because I actually made an effort to make my speech easier for locals to understand, rather than the confusing Portuguese/Dutch/American/RP accent I tended to have when speaking English in lazy mode).

There are a lot of Brasilians in Portugal (about 3% of the population, not counting those who got Portuguese nationality which they can after 5 years without having to give up their Brasilian nationality) and that also includes a lot of kids, so of those kids the ones who came here when they were already 5 years old or older would speak with a Brasilian accent.

In my own experience living in several countries and learning their language, which included picking up their accent, you don't get the accent of the speech you're exposed to a small part of the time, you pick up the one you're exposed to most of the time, so for example my Dutch has an Amsterdam accent and I didn't at all try to pick it up, I just lived there and that's what I heard most of the time from those I spoke with.

I've heard some little kids speaking in accents not from America, and it's been attributed to kids shows being produced by Commonwealth countries (think Bluey, Peppa Pig). But it was clear that they thought the accent was just a "fun" way of talking, and would snap out of it when talking seriously.

Which British accent? Westcountry or Scouse, you probably have a point.

And then you go live the UK, realize that tv uses only a small subset of British accents, and sometimes find yourself wondering “Huh I wonder what language that is?” only to realize it was English 20 minutes after the fact

I have to turn on captions sometimes

Which British accent?

There's the "standard" called RP (for Received Pronounciation) also known as the BBC English, there's the rich people's accent (yeah, rich people in Britain have their own accent) known as Posh English, then there is a poor/working class Londoner accent called Cockney Accent (which outside Britain you often hear in TV series taking place in working class London neighbourhoods or when showing poor people in 19th century London), then there are a number of regional ones just in England (though those are harder to explicitly recognize if you're a foreigner, even if for example you can tell that somebody from Manchester has an accent different from somebody from Essex), then there are the other nations of Britain (Scotland, Wales, Northern-Ireland) which themselves have one or more accents each (I know for sure Scotland has more than one accent since I can notice the difference).

Mind you, I only know this because I lived there for over a decade.

At what point in time? the language is nearly 1400 years old.

Source?

That's absolute horseshit made up by a journalist on a slow news day, by the way

British English is not some monolith and was less homogeneous than it even is now at the time many were coming to the Americas. If this were true it would only be true for a particular region. English outside of the UK also diverged as it no longer followed trends happening there, and regional variations went in sometimes different directions.

Even within the US, English isn't super homogeneous. Look at Appalachian compared to California or someplace. Parts of Louisiana have unique features from Accadian and influence from Spanish.

Really? I thought this was only the case with Quebecois and French

Even Canadians know they are the ones who sound funny.