Greek citizens traveling from the United Kingdom with a national ID card
According to information of the European Commission, it is confirmed that the travel of Greek citizens from the United Kingdom using their national ID card is allowed, provided that the national identity is approved by the country of destination. More information about the ID travel of EU citizens can be found at the attacheddocument of frequent questions and answers of the European Commission in the following hyperlink https://ec.europa.eu/info/publications/travel-documents-required-eu-citizens-when-travelling-uk-1-october-2021_en, as well as in the website of the British government. https://www.gov.uk/guidance/visiting-the-uk-as-an-eu-eea-or-swiss-citizen
With the exception of the cases mentioned in the above websites, Greek citizens travelling to the United Kingdom must present a valid passport.