Britain most likely would have assisted I imagine as they did assist Pedro in reclaiming the Portuguese throne for his brother. Its not out of question that eventually Carlota Joaquina and Prince Miguel would return to Lisbon at some point and if a liberal government is proclaimed, they will look to Spain and France to intervene. Remember the French intervened in Spain in 1823, and Charles X of France would not hesitaste to do the same even if it meant establishing an absolutist puppet regime. This would almost certainly make Britain intervene by providing ships and commanders at the very least.
One thing that an imperial government might do is continue to push for agricultural settlements of Portuguese peasants in Southern Brazil. If I remember correctly one of the last was in 1817 in Espírito Santo with Azoreans. In OTL though the Portuguese were by far the largest immigrant group during the imperial period, there seemed to be a push to settle Germans and Swiss in the agricultural settlements. You might see the government be more concerned with settling peasants from Portugal in the interior of Southern Brazil, as a way to relieve the poverty there.
I think the difference between Britain and Ireland is a bit different, particularly as the majority of Ireland was Catholic and the Irish were forced to pay tithes to the Church of Ireland until 1838, and were most certainly a disenfranchised group. This was not the case of the Portuguese and if anything in the 1820s and 1830s some were still resented for the positions of power they held in an independent Brazil. Into the XX century, a significant number Portuguese who did move to Brazil often were able to occupy positions of political and commercial significance. The two identities were much closer than those of English and Irish as they were usually interchangeable, with Portugal having been ruled by a Brazilian-born Queens until 1853. Until the early 1950s, the overwhelming majority of emigrants who left Portugal headed for Brazil, I imagine if the two remain united, that this will be even more significant. As they are not "foreigners" they might get first choice of lands as they open up, whereas Rio de Janeiro will be even more Portuguese than it was (in 1890 51.2% of the city's inhabitants were either born in Portugal or had a parent who had been).
Portugal's position in the empire would be more akin to that of Northeast Brazil within Brazil, meaning a place of former prestige and grandeur, but now of declining relevance. The rise of gold mining and later coffee would mean that growth was focused in the south and as a result the Northeast never regained its prominence. Initially, Southern Brazil was a backwater, but with the discovery of gold, Rio became prominent as a port and finally in 1763 became the Viceregal seat. Portugal might just end up like a larger version of the Azores, where the populace was largely resigned to rule by Lisbon, but with an ever-growing population dependent on emigration. One thing that Portugal does have in importance is the University of Coimbra, which would remain the place where much of the Brazilian elite was educated until Brazil established its first university in 1920. Porto too would continue to grow in the 19th century particularly with the importance of the wine industry in the Douro Valley, and this would remain Portugal's leading export for decades to come.
The war in Banda Oriental might also go differently, particularly without the Portuguese/Brazilian in Brazil still joined. Again this might be another territory they might want to settle quickly with peasants from the Azores and Madeira. They had already done this decades before in Rio Grande do Sul, Santa Catarina and Paraná so this might be the most obvious course of action.