Hi, I want to iMessage an American phone number from the uk, how do I do this?

make sure you have the phone number entered in international format in your contacts. In settings/phone turn on dial assist. Then enter the US number as +1 (area code, if it has one) (phone number).

With dial assist on the + gives you the dial out code to make an international call, 1 is the country code for USA,for a mobile number it will not have an area code but if it has a leading zero leave it out, then the phone number without any leading zero