If you are a British citizen who lived in Denmark in accordance with the EU rules on free movement before 31 December 2020 and still reside in Denmark, you are covered by the Withdrawal Agreement between the United Kingdom and the EU. Therefore, you can continue to reside in Denmark after this date on terms, which overall correspond to the current terms under EU rules on free movement.
Consequently, if you moved to Denmark before 31 December 2020 and still reside in Denmark, you can apply for equal status with Danish citizens and SU on terms, which overall correspond to the current terms under EU rules on free movement, for instance as a worker under EU law.
Be aware that if you have resided in Denmark before 31 December 2020, you have to submit an application for new residence status under the Withdrawal Agreement to The Danish Agency for International Recruitment and Integration before 31 December 2021. You will receive more information directly from The Danish Agency for International Recruitment and Integration.
As a rule, if you moved to Denmark after 1 January 2021 or later, you cannot apply for equal status with Danish citizens and SU on terms, which overall correspond to the current terms under EU rules on free movement.
Visit ufm.dk for more information on Brexit.