The UK left the EU on the 31 January 2020. There will now be a transition period until 31 December 2020. During the transition period there will be no changes in practice for citizens and businesses in the UK and the EU.
Travel to Sweden will remain the same as before brexit during the transition period, until 31 December 2020. UK citizens can still travel to Sweden with only a valid British passport until 31 December 2020 (no minimum validity required).
Please note: if you hold a UK Certificate of Travel, you will need a Schengen visa to travel to Sweden.