I just did a check for my next block of days off Perth to London 20-30 April (was and are flexible).
Singapore Airlines won.
Depart 22 April [Sun] at 14:05 on a B777 (nice time) arrive SIN 19:35 with about a 4 hour break. A bit of a stretch see what's on show in the terminals and beer in the Cactus Bar maybe a snack.
Board the A380 for London at 23.30 after a round of snacks and a beer or two, there is a chance of some sleep on this craft and the airline economy standard. Arrive London at a reasonable 6:00 in the morning [Mon].
Now for the return - Depart 22:05 on the 30th [Mon] on the A380 again after a snack and a couple of drinks there is a chance of some sleep early on when it is dark. You will arrive at SIN at 16:10 on the 1st of May [Tue], I chose the longest layover that was actually the cheapest (free) but would have anyway. This is a 13 and a half hour layover in Singapore.
I booked a $139 room at the Transit Hotel in T2 for 12 hrs inc breakfast.
So on arrival at Singapore it will be to check-in and grab a shower and change of clothes. Then a beer around the pool and go get a meal at one of the many outlets. Depending on how the previous flight went I can decide on getting 6 to a good 10 hors of sleep in the room.
Get up have breakfast (included) and head off for boarding the 777 for a 7:40 departure 2nd May [Wed] arriving about lunch time the same day in Perth.
Cost $1,163.73 + $139 room
Pretty sure I could pay for the beers in Singapore with the savings from the direct flight cost & turn up for work on Thursday.