I've had good luck booking 4 seats together on EY using VA points. Search using Etihad's reward seat tool:
Book flights with Etihad Guest Miles - Etihad Airways
VA availability has historically matched availability with what is showing on EY - unfortunately that is no longer the case. Also EY redemptions go up in price from 1 June...
A guide to SQ upgrades can be found here:
Singapore Airlines business class upgrade guide - Australian Business Traveller
Thanks for your help. I have tried Etihad site to do a dummy check and can't find any days in March/early April where I can get 4 guest business seats. That's trying Abu Dhabi to Rome/Zurich/Munich/Paris/London. So unfortunately unless their availability gets better, even before trying to get virgin to match it, I think we are out of luck.
Pretty happy with the Singapore upgrade option though. Will only cost us ~$1700 per person for Flexi economy from Manchester to Brisbane. Then only 74587 velocity points to upgrade the long leg once converted and after the 15% discount. 109000 velocity points and $500 of taxes on the way over.
So all up per person it is ~183000 velocity points and $2200 for return business (excluding Singapore to Brisbane). Or can also upgrade that leg for about an extra 12000 velocity points. And I would get 280 status credits which should be enough to keep my gold with my other credits. At worst I would need another 20.
This option is way better than spending $2300 per person for PE on the way back.