Most comfortable economy class to London would be SIA A380 upper deck. There is a small economy cabin located at the rear of the business cabin which is very quiet.
Not much further behind would be the Qatar or Etihad A380s all the way through. Product is brilliant and service mostly top notch.
I have not flown Qantas internationally in 30 years and have no intention of doing so. If they bring their A380s up to QR/EY standard then I’ll give them a run.