I’ve had this a few times and that workaround is correct.
Move your Avios firstly from BA to QR and then do your booking.
That said, I’ve started to make the majority of my Avios bookings on QR, via the BA website now. Mainly because if I need to cancel, refund and rebook, it’s a far slicker process via BA in terms of getting the money/Avios back, than it is when booking via QR.