No, it is not similar to Singapore - New York. That is an A350-900ULR with about 160 seats.
Sydney - New York is about 400NM longer (about an hour longer), and Sydney - London is about 1000NM longer (over 2 hours longer), and Qantas want 300 seats.
No-one has said QF cannot do it, QF are saying they can do it, but they need a modified A350-1000ULR to do it. The 900ULR Singapore uses won't cut it.
What is stopping an agreement? Well, so far Qantas have asked the pilots to make concessions and the pilot negotiators have not yet agreed to these concessions. The details of the negotiations are not public, but in layman's terms you could probably put it this way: Qantas want the pilots to do more for less or they have threatened not to buy the aircraft. They have also stated this numerous times in public press releases.
Now, this is the sticking point: Qantas have claimed that the business case won't stand up if the pilots don't agree to these concessions.
This is where Qantas have made themselves look very foolish, because anyone can see that if the project is built around such tight margins that a few dollars out of the pilots' pockets will make or break the business case, then it most certainly should not go ahead. Or, as is obviously the case, the business case presented to threaten the pilots is a complete fabrication and nobody believes a word they are talking about. The obvious truth is that Qantas are putting a lot of pressure on pilots to reduce costs in general and are using such threats as a negotiating tool. The pilots don't like being lied to, and this probably isn't helping the negotiation.
Qantas management is very excited about this project. However, most pilots actually aren't all that excited about working more fatiguing duties for less reward, not to mention the lack of cooperation from the Company about fatigue management. The pilots have already called their bluff once by not agreeing to the concessions by the Company's deadline of last week. So the Company have created a new deadline of sometime mid-February. We'll see what happens over the next 2 months.