A flat-rate U.S. Postal Service Priority Mail Express International envelope mailed from the United States typically takes between three and five business days to reach an address in the United Kingdom. USPS does not guarantee a delivery date for international mail.
USPS offers Priority Mail and Priority Mail Express envelopes and parcel service to England, Ireland, Scotland and Wales. It costs $42.75 to mail a 12 1/2-inch by 9 1/2-inch flat-rate envelope from the United States to the United Kingdom, as of May 2015. A large Priority Mail Express International flat-rate box that weighs up to 20 pounds mails from the United States to the United Kingdom for $80.50.