Do not underestimate how different was life before fast long distance communication and travel.
About Brazil: ~1820, Brazil had 6M people, 3M free + 3M slave. Portugal had 3M.
When Brazil was raised as part of the kingdom, there were deputies representing Brazil, but very few. People noticed that it was not fair, but fairness would demand that half the seats be given to Brazilians. Do you think a Portuguese would easily accept that half of his lawmakers come from the colonies? Besides that, it would take months for a Brazilian lawmaker to communicate effectively with his electors.
I am not sure about pops in the Spanish, French or English empires versus the metropolitan pop, but I guess it would be very relevant.
About US: everybody heard "no taxation with no representation", and this has a lot to do with the original colonial pact (Edmund Burke wrote a lot about this):
- US colonies would not pay tax
- US colonies would import/export only with England (monopoly)
- US colonies could not build some kinds of manufactories.
This happens because it would be very difficult for the King to tax a distant colony. Again, because any dispute would take months to be solved. It is easier to impose the monopoly and tax any commerce in the UK side.
Some people say that the taxes imposed by the UK before US independence were not so high, but the amount was not the point: any tax was too much if the monopoly and manufactory restrictions were still in place.
And on the UK side, many people depended on the monopoly. It is not very surprising that after the wars commerce UK-US resumed in the XIX century, as the trade connections were already well established.
If suddenly there were fair representation (and taxation) for all the colonies, then how would all this change?
Some general comments:
Do you know the history of the cooperative of pig producers that started to help its members to produce chicken too? Suddenly there were conflicts over allocation of resources to pig problems against chicken problems, and the cooperative split. A parliament representing colonies and metropolis would have similar problems. Too many diverging interests. At least some kind of federation with local parliaments would be needed.
Some colonies are too small to be independent. During the independence of Brazil, there were people on the Grao-Para province (amazon river) and Angola which wanted to go with Brazil and others to remain in Portugal. But independence was not an option, as Angola depended on comerce, and Grao-Para, even in 1870 still could not have a profitable boat transportation system: they could not be independent with no subsidy from a metropolis. (Eventually Grao-Para went with Brazil, and the independence treaty stipulated that Brazil did not have a claim on Angola)
A similar reasoning is true for many small UK colonies. If a relative lack of representation is the price to pay for the economical and defense support from the metropolis, then let it be.
And if a colonist is not a citizen in exactly the same way as a metropolitan, then strange convoluted citizenship, immigration and passport rules such as in the XX century british territories naturally emerge and even make some sense.
Another related XX century anecdote, to show that some people had reason in mistrusting some modern independence movements:
When Angola and Moçambique got independent, there were white or mixed race pops, some of them did not even remember when their antecessors come from Portugal to Africa. And everyone there could choose Portuguese citizenship and emigrate. Most of the black people fell for the communist promises of a new popular black government and stayed. But 800K people, mostly non-blacks, emigrated back to portugal, and many of them did not have any known relative in portugal anymore. This was almost 10% of portuguese population.
I have known 2 white (not really nordic-white, may be they are mixed race) families who at first decided to stay, as all their business and families were in Angola and Moçambique.
A shoe-maker in Moçambique had a two-store house, with his shop in the lower floor. After independence, the commie government come and stated that his house was now property of the "shoe makers communist association", that all his profits belonged to the government, and he could only take a fixed salary. As his fixed salary was ridiculous, he had to sell out of the books, and heavily bribe the commie who come every week to collect his profits, to turn his eye elsewhere. Then, he decided to leave to Portugal, still in time to collect his Portuguese citizenship, and start life from scratch. His son is today university professor at Coimbra.
A orange farmer: six months after independence all was nice. Then the commies come in tactical trucks, inspected the farm, and declared that the people committee would decide what to do. Life went on for some months. Then, the family went to a neighbor farm for a birthday party. Suddenly, their right-hand man, a black African, come pretty hurt and blooded: "the commies come, they killed every worker. I hide among the corpses, and when it was dark I ran. Lets all of us run! They know about this farm too!" So they went all to Namibia, where they were segregated from their black savior due to Apartheid laws. When moving, they could look back to see smoke raising from their farm: the commies just burned everything. They also had time to emigrate to Portugal and claim Portuguese citizenship. They regret very much that looking at Google Earth, their once lovely farm is now only savanna and ruins.
Do you really would want a peace that would grant parliament seats to those independence parties?