in 1807, Britain is unquestioned master of the sea. Actually, they had been for decades, but after Trafalgar, there is zero doubt that France has no way to take advantage of Brazil. The best France could hope for is to capture Portugal's fleet along with the royals, but even then Britain has the means to eliminate any threat that might pose.
So, the question isn't whether Britain denies French/Portuguese access to the colony, but what form it takes. They're still licking their wounds from screwing up in Buenos Aires, so I doubt they go the take over route. They can fairly easily block the main ports and/or encourage the Brazilians to not take orders from the victors in Portugal. Dunno if they'd go so far as to foster independence. Depends whether there's a resistance group in Portugal, or whether they all just roll over and accept being partitioned into 3 and given new masters. You can look for clues from Spain and it's colonies during the Iberian war. France didn't have access to the colonies, and Britain didn't foster independence. After the war, presuming same outcome as OTL, I'm guessing Brazil goes the same route as Spanish colonies: a path towards independence, with Brazil splintering into several countries. Even with the King in Brazilian residence, there were still separatist movements. And, if I recall correctly, the Amazon region was a separate political entity from the rest of Brazil in colonial times, until Joao merged them together when he arrived.