I would put in my VFF if I were doing the flying.
SQ as an airline is very good, but you do need to earn and burn fast them points, to make it work worthwhile, as the miles do have a life of 3 years, and not extedable, but due to covid, they extended the life their KF miles, but I believe thats ending soon.
For VFF, even if you don't make to SG due to the 8 sectors, even having 4 sectors and PS is still very good, and your VFF points don't expire, so long as there is activity, ie, going to a 7 eleven and buying something small, and putting your VFF # via 7 eleven app onto the scanner.
To transfer points from KF to VFF, you loose 35% of the miles transferred.
SYD - SIN - LHR in one trip, is not going to reach Elite Gold in KF, need to do a double of that, ie, 50,000 miles in J to make it worthwhile.
UA is also a VFF partner, so easy to keep VFF points alive that way.
The J trip will give you J lounge access and the J seat anyway.
To summize, I say, keep in VFF unless you have more and more flights in a year, with SQ J/F, then its worth to go with KF PPS.