You currently have enough Qantas points for one, one-way flight to London in Economy. Depending on which airline you fly, you may or may not have enough points for a Premium Economy flight. Even if you move all your Flybuys points over to Velocity, you won’t have anywhere near enough points to get to London using Velocity points.
I would suggest that you’re still at the stage of accumulating points rather than using them. I can think of a couple of possible strategies:
Either, pay for your August 2019 flights. Now is the time to get them relatively cheaply.
Or (the riskier option), continue to accumulate points until, say, March or April next year, and then try to book reward flights for August. Use the resources of this website to fast-track your points accumulation. Particularly if both you and your partner work, and you can both do some serious point hacking, it’s not impossible that you could find yourselves with, say, another quarter of a million points quite quickly (this totally depends on your financial situation, income, willingness and ability to sign up for credit cards, etc). That would put return tickets in Premium Economy in the ballpark.
Which strategy you take depends entirely on how happy you are with the risk that you might not be able to find reward seats 4-5 months out, which will mean paying for tickets which almost certainly will be more expensive by then. My advice is that if you need to fly on or near specific dates, don’t risk it and pay for the tickets now. Use your points for another trip later. If your dates are quite flexible, thus increasing your chances of finding reward seats, you might like to take that risk.
All the best!