Another option you have is if you don’t wish to travel is that you can apply for a Future Travel Voucher which is valid until April 2022, if they extend the travel period until end of September. You would get the full value (no £35 cancellation fee) and the Amex Voucher validity will also be extended for travel to be completed by 30 April 2022) as this is part of your voucher.
TPG UK reported a success story and I have just done this yesterday as well with a voucher which otherwise would have expired before I could have used it sensibly.
For now I would sit tight and not do anything until they either cancel the flight and you can rebook at any other date if you wish or get a full refund and the voucher reinstated or if they extend a Future Travel Voucher into September go with that option.
I hope you get the result which works best for you.