US Army Warrant Officers ARE NOT equivilent to Brit Warrants.
As a WO1, I'm not a Commissioned Officer but I am an appointed Officer (by Warrant) and a member of the O Club. On promotion to CW2, I will receive a Commission, so I'll be a Commissioned Warrant Officer.
And on saluting:
According to regulations, while in uniform, enlisted personnel should salute any Commissioned or Warrant Officer in any branch of the US Uniformed Services:
-US Army
-US Air Force
-US Navy
-US Marine Corps
-US Coast Guard
-US Public Health Service Commissioned Corps
-National Oceanic and Atmospheric Administration Commissioned Corps
That would go for all other friendly forces too.
Again, why do Brits have a problem with this?