In order of value:
1. Use them for business class seats to Europe - SYD-LHR has the most seats per day, so hopefully the most availability, but sometimes other destinations have availability. Book a flight at the far end of availability - about 350 days out - today, 17 July 23, I can book a flight on 6 July 23 - this let's you get a seat in peak periods, just has to be one year away.
2. Roll them over for 6 more months (Will cost you 1200 points per 10k extended - a 12% haircut, and only for 6 months, so don't do it if you're not going to use it. Only really handy if you want to book a flight 12-18 months away from now, obviously.
https://www.singaporeair.com/en_UK/us/ppsclub-krisflyer/forms-and-fees/service-fees/
3. Transfer them to your
Virgin Australia Velocity account where they will not expire, as long as your Velocity account remains active. Exchange rate is 1.55 KF to 1 VA point (65%, so 65k VA points per 100k KF transferred) A 35% haircut but better than losing them all.