Using Britain as your example is probably not the best look. However, it will be a problem even if elections are reinstated because he already banned opposition parties.
This way he can “reinstate elections” and be praised as a pioneer of democracy without any real risk to himself because all his political opponents are already outlawed.
Of course, that is all under the assumption he would still be in power after the war.
EDIT: Also, how long is “the duration of the war”. Until a ceasefire? Until one side surrenders? Zelensky would probably say until the unconditional surrender of Russia, an event that is never going to happen.