Its up to you - if you want to cancel and get the avios, voucher and fees back (minus £35pp), you can - but you need to call. The 241 voucher will be returned with its original expiry date, and you can then re-book online as before.
A so-called FTV is really just a credit note - they hang on to everything and you then use that credit for a new booking - it has to be another reward booking and for the same people. If the 241 voucher is soon to expire, the credit route keeps it 'alive' until Sep 23. You don't pay the £35 fee but you do need to call to re-book, using the credit.
Lots more info in the dedicated threads:
BA American Express companion voucher | master thread
and
BA Covid-19 Flight cancellations, rebooking, and refunds | Help and advice thread