That SIA direct flight to JFK was on an A345 that was all business/first. Only 90 seats on board IIRC. If that didn't work between two of the wealthiest and most financially important cities in the world.......
Indeed, and any LHR-SYD would also have to be F and J only, but there clearly isn't the appetite for this. How much time would it actually save over a one-stop, 2-3 hours?