The U.S. standard for defamation could hardly be lower. New York Times v. Sullivan basically prevents anyone who can be construed as a "public figure" from suing for defamation.
Britain's libel laws are draconian by U.S. standards. People still win substantial verdicts in defamation cases over there. And IIRC they still have a censor.
If Kelly indeed is releasing her book there first, she's counting on the liberal British press to slime Bush, because as a foreign head of state he (as a matter of P.R.) can't bring a libel suit against her. I also wonder whether the British courts would have jurisdiction over her (as opposed to the publisher.)
You're right, I should have better described what I meant by "standards" - the standards by which one seeks to prosecute for defamation or the standards by which one is defended against claims of defamation!!!! Whew, that wacky lawyer legal lingo, again......thanks for making it clear!!