Best answer is .... Yes you are supposed to, I know many who have come back from such countries and not stayed at home for the 14 days.
One instance is my own Son, was in Germany when this kicked of again, went to Holland because they were not on the list, overnight they were, so he went back to Germany, drove to France non stop, not allowed to get out or stop anywhere, got turned back from the ferry, even though his ticket was with them, otherwise quarantine, drove on to the Euro tunnel, French border force checked everything over, had to get out of his vehicle, spoke with border people who were more interested in the mess Johnson has made, so they said, on the train back to the UK no quarantine if you come via the tunnel.