It is still about 26 hours before tomorrow's flight departs - by 24 hours before departure when checkin maybe there will be some tinkering and upgrading that may free a few seats into economy class

If you do happen to get denied boarding due to a full flight, then hopefully the GBP520 might soften the blow if delayed 4+ hours

There does seem to be 8 seats in economy that are showing as available from SYD-SIN and 16 from SIN-LHR , so some seat shuffling should provide some seats for the through journey