It does look like SQ is doing a one off SYD-SIN-LHR on Sat 11 April.
It's bookable on their website...90 minute connection but still 2 standard flight numbers SQ232>SQ322 (no deplane apparently).
It's being promoted by the British High Commission in Canberra, although helpfully when booking it still carries a message that only Singaporean residents are allowed to transit...not very confidence building!