I had two F seats on CX MEL-HKG all ticketed for August. Earlier this year CX removed F from all the Australia flights and now we have been downgraded to J for that segment. Quite disappointing since I routed via MEL and redeemed an F award specifically for the purpose of F on that route.
So if QF remove F from the route, you may well find yourself in a similar situation with J seats for the "price" of an F award.
Did CX not reticket you on a J award even though all of your travel was now apparently in J?
To the original OP, if you have F award tickets on QF 29, I would insist when contacted by QF that they reaccommodate you on a F flight ex SYD, regardless of whether there is techically award availability at the time they contact you to reticket you. This may not be their first offer though.
As an altnerative, insist on CX J to HKG and then CX F to LHR on the service that leaves on the evening you would have travelled on QF.