QR do not market premium economy fares so you are trying to do something they do not offer. It is possible they are quoting you the difference between a basic economy and maximum PE fare, or possibly even business. If BA have some sort of special upgrade offer for the price you quote then QR won't be able to offer that. If you don't want to pay what QR are quoting then you may have to stay in Y, or cancel and rebook with BA.