Effective January 1, 2021, the United Kingdom is no longer a part of the European Union (EU). Therefore, the Open Skies Agreement with EU countries no longer applies to the United Kingdom.
From the U.S. General Services Administration Fly America Act website: "'Open Skies Agreements' are bilateral or multilateral agreements between the U.S. Government and the governments of foreign countries that allow travelers to use foreign air carriers from these countries for government-funded international travel.”
Please visit the Fly America Act website for more details.