Americans are banned from the UK now.
Strange I have never known anyone be refused. Not even serial killers. What have you done?
It all depends on WHY your application was refused ; but certainly if you simply turn up I'm fairly certain you'll just be sent back on the first avilable plane ; indeed I don't think the U.S. authorities would even let you board a plane for the UK in the first place. You may get some idea from http://www.ukba.homeoffice.gov.uk/visas-immigration/
THEY LET EVERYONE ELSE IN.....
That seems rather harsh as they are either letting in thousands of immigrants here due to "staff shortages" or due to total incompetence, having lost track of many thousands more! Seeing as you produced proof of financial support for yourself, unless you have a criminal record, I can't see a problem. Wait until you get some details from the authorities and see what they say !!