United Kingdom: Background, Brexit, and Relations with the United States [Updated April 3, 2020] [open pdf - 1MB]
From the Document: "Many U.S. officials and Members of Congress view the United Kingdom (UK) as the United States' closest and most reliable ally. This perception stems from a combination of factors, including a sense of shared history, values, and culture; a large and mutually beneficial economic relationship; and extensive cooperation on foreign policy and security issues. The UK's January 2020 withdrawal from the European Union (EU) is likely to change its international role and outlook in ways that affect U.S.-UK relations."
CRS Report for Congress, RL33105
Congressional Research Service: https://crsreports.congress.gov/