Yes you can, but you will have to show the Brazilian passport to the airline when you check in to fly to the US. You can show both passports to the airline. They'll send your Brazilian info to the US. They might also send it to the UK, which is not a problem.
In fact, there's no need to show the British passport until you check in for your return flight to the UK, unless someone asks you about your right to be in the UK.
The only thing to worry about, really, is that there could be problems if the UK receives your Brazilian information when you fly out of the UK. If that happens, they will presumably be unable to match the exit record with an entry record for your Brazilian passport. This is why you might want to show both passports at check in.
But I do not know whether airlines are capable of sending different document details to the UK and the US for a flight from the UK to the US. Even if they agree, the check-in agent might accidentally send the Brazilian information to the UK, so it might make no difference that you've shown your UK passport.
In the end, if they do send your Brazilian information to the UK, nothing bad will come of it. You are not doing anything wrong. The UK does not hunt people down when they have unmatched exit records, and, even if they did, and they found you, you would simply explain that you are a dual citizen and checked in with your Brazilian passport because you needed to use it at your destination.