The U.S.'s international calling code is: "011," so you will begin by entering this number to dial out. If you are calling from another country, you will need to find out its international dialing prefix.
Next, enter the code of the country you are calling. In this case, it is "44" for the U.K.
Enter the city code and the local number you wish to reach.
In total, you will dial "011+44+XXX+XXX+XXXX."
Do not dial "1" first as you normally would to dial long-distance, as the international calling code replaces this "1." If there is a zero between the country code and the city code for the number you are dialing ignore it. Dialing "00" will connect you with an international operator who will be able to help you if you have repeated trouble.