Answer: The United Kingdom is a Northern European country that chose not to be in the European Union after Brexit in 2020. Context and details:

• Northern Europe includes several countries that are not EU members, such as the United Kingdom, Norway, Iceland, Switzerland, and others, but the United Kingdom is the notable Northern European country that formally left the EU in 2020 following a national referendum and subsequent withdrawal agreement.

• Norway, Iceland, and Switzerland remain outside the EU but participate in various EU-supported frameworks (Norway and Iceland via the European Economic Area or bilateral agreements; Switzerland through a set of sectoral agreements). These distinctions often lead to confusion about “Northern Europe” and EU membership, but the UK’s case is distinct in that it fully exited the EU and is no longer part of the EU’s internal market or customs union.