When they see that your education and first jobs were in the UK they'll know theyre dealing with something they might not understand, and will ask if they want to know more about your education and qualifications. Dont explain it on the Resume. They'll ask that stuff at interview if they want to see you based on your work experience. I too have to deal with that issue; but, like missy, its the other way around. It would be good to keep copies of your scores / degrees / certificates handy and take them with you to interview incase they ask.
The biggest thing I have to say about this topic to anyone writing a US resume for the first time is : CVs are NOT acceptable for most jobs. CVs are for people who are doctors, teachers, scientists, etc., or for people who are well published. Resumes come in all sorts of different shapes and sized so you should check out a "How To" on the web to find out what kind of resume is going to go over best with your target industry.