1 plane cannot fly SIN-LHR and back in 24 hours. If they want to serve the same flight each time, they can only use the plane 3 days a week. SQ would need 2 planes to fly daily.

If Singapore gets the second plane some time before third, it is thought that they would continue to fly one daily SIN-SYD flight and use the second plane for 3x weekly SIN-LHR till they get the third plane and start daily SIN-LHR.