The food on BA9 BKK-SYD in F is truly shocking and the service can be sub par. The lounge experience n BKK is also nothing to write home about even if it is perfectly adequate. I know that SYD-BKK is effectively a day flight but it does come with better catering and access to the quite superb QF F Lounge at SYD. If I had to choose only F in one direction I would go for SYD-BKK on this basis even if this feels rather counterintuitive.

Apart from slightly more seat room and better drinks there is little other effective difference between J and F BKK-SYD. Food was actually better in J on my last trip.