The Chairman (legally) can only resign to the full council. Unlike a Councillor resignation which is addressed to the Chairman.
If the Chairman resigns, the next item on the agenda, is automatically by law, the election of the Chairman.
As this seems like a planned resignation, (they want to say thank you, etc) then after the approval of the minutes etc, you can have "Statement from the Chair", and the next agenda item "Election of Chairman", the Clerk will want the meeting to operate smoothly so they would normally encourage this.
UNLESS, if your September meeting is the stand-in Annual Meeting, in which case the election of Chairman must occur first (and legally, (more in theory than practice) without a statement from the outgoing chair.