In terms of the demand, I’m sure there are now more people who will consider Australia, given it is easier to get to. But also, previously you had no option but to stopover enroute, hence making the ME3 and CX equally attractive to UK passengers. However, if BA and Qantas can serve the market direct then they will claw back some of the connecting passengers and will have an advantage over the ME3.
But yes the 777-300 with 72J would seem a bit too much for the SYD route, but I don’t know maybe they want that.
And do note I didn’t say BA should expand in Australia, I asked would they.