Since you presumably are entitled to compensation for the original flight cancellation I would add the request to have your Bronze status renewed as a note to the case number. Explain that you originally booked your flights to renew status and had BA not cancelled your flight you would have renewed. Also say you've tried calling the Bronze line and can't get through. (if you have a call log list the dates and times or ideally upload a screen shot) Include the details of your revised flights you will fly. I would be very surprised if BA did not extend your status in the circumstances although best to be prepared for a rejection. Good luck!

Edit: and don't forget you won't actually drop to Blue until 1 September. - you've just run out of time to earn the required TP to automatically renew.