You have 110k SQ miles to burn for each passenger.

If you want to fly SQ, your best bet is IAH-MAN for 72k miles (saver), PDX to IAH with the remaining miles and probably a cheapo paid flight from MAN to London City or GTW.
The availability on IAH-MAN is usually quite good.

Otherwise, try your luck on UA or AC flights to London. You can check availability on UA and then call SQ to book.