I'm not sure if I've got this right, but AFAIK
Council Tax and Electoral Register are supposedly not linked, or are they ?
Let's say that Joe Bloggs is sole owner/occupier of a house, he gets the 25% discount on Council Tax, and is the only person on the Electoral Register at his address.
Then a friend of a friend, Jan Kowalski from Poland, gets a job in the town where Joe lives, and Jan needs somewhere to live. Joe says Jan can live at Joe's house for the foreseeable future (and being a philanthropist, only asks for a small sum of money to cover increased use of gas/elec/water, and also to cover the extra 25% increase on Council Tax). Joe goes online to remove the single person discount, and finds that he has to enter the name and previous address of the new occupier, so he enters Jan Kowalski from Poland. Joe does not enter Jan on the Electoral Register because Jan is not eligible to vote. Then Jan's brother, Jakub, also gets a job in the area, and having another spare room, Joe offers it to Jakub. But this time, Joe has to do nothing, because full council tax is now being paid, and as with Jan, Jakub is not eligible to vote.
It begs the question, why the feck does the Council want the name and previous address of a new (2nd) occupier when declaring removal of the singe person discount ?