First, lets have a form of diplomatic agreement, not likely, but possible. As a result, Britain closes most of its relations with the USA.
This causes no British support in Iraq, which means it would take longer to subdue the country, and supposeing Iraq really has WMD, they could launch attacks on the East Coast, either by missiles, or simply hiding a nuke on a merchant ship, and letting it go on arrival.
The economic disaster of this would cripple America.
Maybe in the near future, China reforms into a democracy, however this causes heavy economic difficulty and brings the country to a halt.
Britain meanwhile, streangthens its alliances with the CoN, and thus, gets more trade from them.
Give or take a few years, and the Brits could be in Washington's position right now.