SQ never operated YYZ-LHR, they operated YYZ-VIE-SIN.
AC and OS had the rights to VIE which neither were exercising. The Government's position was that they would not grant 5th freedoms ex-Canada to countries other than the USA in circumstances where more than 50% of the traffic ex-Canada was not destined to the carrier's home country. Since SQ was carrying more than 50% of ex-YYZ traffic only as far as VIE, their 5th freedom was revoked. Even SQ couldn't make YYZ-SIN work.