I would assume the US would warn it's allies about the bomb drop to avoid friendly casualties. Were the locations chosen because they knew no allies were on the ground, and therefore they wouldn't need to warn the allies? Or did the US send out a warning about the power and destruction about to be unleashed?
The UK worked with the US on the project and it was agreed they would give consent before use (as part of the Quebec Agreement). They had representatives in the United States who were told and consulted.
The fact that a bomb was to be used soon was hinted at by Truman at the Potsdam Conference.
In neither case were operational details — locations, times, etc. — given. I am not sure why you think there were "friendly casualty" possibilities — no Allies were on the Japanese mainland in early August of 1945 (there had not yet been any invasions). There was a concern about attacking cities with POW camps in them (and indeed there were some "friendly" casualties at Nagasaki), but other than that, there was no further "friendly" issues.
The US did not send out any general warning; the bombing was meant to be a shocking surprise and very intense secrecy was kept.