Did it for a few years while working there (in y and on different airlines / timings). So this was focused on jet lag rather than enjoying the flights. Agree with others arriving in the morning is hardest timing but that's what you have.

Set watch to new time as you leave. I would get up early on departure day, since your departure is 8.30 am new time, I would eat in the lounge and get straight to sleep for a few hours and wake myself up after ~6 hours so I can sleep again on the SIN-SYD leg. So in Y i would take a sandwich with me and eat the second meal. As said use the lounge at SIN and eat enough so you can ignore food on the SIN-Syd leg and just sleep. The first day will feel crappy by 3 pm so get early check in if you can but do not be tempted to lie down, use a swimming pool or walks in the park to keep awake.

Coming back you leave SYD on a 'day flight' on your new UK time. If you can cope tired then try and stay awake for all of that flight, eat, lounge at SIN and then sleep on the second flight. I never manage to stay awake so I ignored the first meal on SYD-SIN and tried to get 4 hours rest, then wake myself up about 3-4 hours from SIN, eat in the lounge and the first meal on the SIN flight and then try to sleep again.