It all depends on where your allegiances and points balances lie. It’s a little unclear to me as you’re talking about redeeming with Qantas one way and then Singapore airlines the next, so presuming that you have a ton of both and no particular allegiance as to which one you spend I think I can help.
If you’re looking at flying East Coast of Australia to London or Paris in an A380 there are really only a handful of options. I don’t blame you as the offering is vastly superior to anything I’ve encountered in my travels previously, however it does restrict your airline choices. Basically you have Qantas running their A380 in and out of London, Emirates who will do an A380 to both, Singapore who will do an A380 to either and Qatar who will do an A380 to either.
Out of those carriers the pick is definitely Singapore as it offers the most luxury, followed by Emirates as it also offers a private partitioned “room” for lack of a better word. I would still rate Qantas as preferable to Qatar in terms of the layout, just as I find that the Qatar first class layout doesn’t offer as much privacy as I might expect, especially if you were paying full fare for the seat. You might notice that Singapore stands out amongst those four, with all the other three being redeemable using points through Qantas Frequent flyer.
The bad news is that First class redemption seats are very very seldom made available through Qantas, Emirates or Qatar. I’m not as well versed with the Krisflyer situation, however with the other three they will make a small amount of First class seats available for redemption 330 days out from departure. If you’re organized and you only need two you might have a decent shot at grabbing them when your dates come into range. If you can’t get them however it’s not good news I’m afraid, as the airlines will generally only free up more award seat space in the lead-up to departure (30-60 days beforehand). If you’re a couple going for a spot of luxury then I’m guessing you probably wouldn’t fancy your chances of just seeing if some drop in in the meantime as you may miss out entirely.
The back up option which I would recommend is take what you can get in Business and then use points to request an upgrade. Unless you’re travelling over super peak season (i.e. December, early January or the European summer) the first class cabin is rarely full, so your chances of an upgrade to first with your points are reasonable. It’s not a plan A, but if you try your best and can’t get a first class seat when they become available 330 days out but you can get a business one, possibly worth a shot securing something in the knowledge that you’ve got at worst business but a probable first upgrade. Bare in mind however that most airline loyalty programs will only allow you to use their own points to upgrade i.e. QFF points are good for only Qantas, and not Emirates or Qatar even though they’re oneworld.
If points are absolutely no obstacle and you don’t mind getting stuck in the luxury or first class for a little longer, perhaps consider some interesting routing to get the availability you want? For example if you have no luck getting redemption seats to Europe, try getting the QF A380 to Los Angeles and then link up with one of British Airways A380 services across to London which you can also redeem using QFF points. It would only extend to a total travel time of say 26 hours instead of 22 hours stopping in the middle east, so worth considering.
Hope that helps!