I have a single entry Type C Schengen visa (Sweden). While in Sweden, can I visit Italy or Germany and return to Sweden before I return to the UK?
Sweden, Italy and Germany are all part of the Schengen Area and you will effectively not undergo Border Control within the Schengen Area.
Since you have a single entry visa, you are allowed to enter the Schengen Area only once and visiting Germany or Italy does not constitute as leaving it.
So, effectively you can enter the Schengen Area in Sweden, then travel to Italy and Germany and then return to Sweden. After you are done with your trip, you can exit the Schengen Area to go to the UK.