These early 1970s "record flights" by 707s of the era were typically positioning flights with no load and thus fuel up to maximum MTOW. Wardair did a comparable one at the time between Honolulu and Gatwick. However such an empty leg eastbound from Gatwick to Singapore seems unlikely when the contract was emigrants to Australia. I believe the emigrant contract also often positioned out elsewhere in Europe to pick up a load - Belgrade and Athens were common points - from where they stood a better chance of making Singapore nonstop.
Did B Cal go right through to Australia, or did Qantas handle things on from Singapore ?
Emigrants from London were also assigned as 'fill in' load on the oddball once-weekly Qantas 707 flight that routed through Bermuda, Mexico City, Tahiti and other points, which was never particularly successful for loads, extremely expensive to operate with the long crew slip durations, and must have made Australia seem a very long way away indeed.