I would not trust that a GPU will clear on these flights unless there is confirmable inventory available at time of booking.

How much are you planning to pay for a SYD-LHR round trip on UA? Last time I looked, they didn't really publish competitive fares in that market.

It might almost be worth buying J on another carrier. If you do want to fly UA, consider buying a B fare, which is not that much more expensive than their other fares in the market and which carries a 50% PQM bonus and somewhat better upgrade odds if you must waitlist.