The cost of mailing a letter to London from the United States depends on the size and weight of the envelope and the mail service used. As of July 2014, a first-class letter that weighs 1 ounce or less mails from the United States to London for $1.15.
The cost of sending a letter to London utilizing USPS Priority Service starts at $29.25 as of July 2014. Priority Service offers customers delivery to London within six to 10 business days. Priority Express Service starts at $35.25 with a three- to five-business-day delivery window. Letters can also be mailed to London using Global Express Service, which starts at $51.50 and offers one- to three-business-day delivery.