WASHINGTON—The U.S. State Department and the U.S. Centers for Disease Control and Prevention (CDC) both issued on Monday their highest warnings against travel to the United Kingdom because of a rising number of COVID-19 cases in that country. Each raised the UK to “Level Four,” telling Americans they should avoid travel there. “If you must travel to the United Kingdom, make sure you are fully vaccinated before travel,” the CDC said in an advisory, while the State Department said: “Do not travel to the United Kingdom due to COVID-19.” In May, the U.S. government had lowered the UK to a “Level 3” advisory rating. COVID-19 cases are rising by more than 50,000 a day in the UK and hundreds of thousands of Britons are being asked to self-isolate for 10 days. The United States since March 2020 has barred nearly all non-U.S. citizens who have recently been in the UK …