Here’s the address for the local British Consulate-General:

British Consulate-General
Georgia Pacific Center, Suite 3400
133 Peachtree St. NE
Atlanta, GA 30303

Their website says they’re open to the public from 9 am to noon and from 2 to 4 pm, but they may be more skittish today. Flowers can probably be left with security in the lobby (I’m guessing).

The address for the British Embassy is:
3100 Massachusetts Ave.
Washington, DC 20008

If you’re worried about a friend or relative traveling in London, the US State Department hotline is 1-888-407-4747.