Have you checked YYJ-SFO-LHR on UA? While UA Y isnŽt as good as AC Y, youŽd not only save the 40k miles, but also hundreds of dollars in fuel surcharges. YYJ-SFO tends to have decent availability IME.
You should also check YYJ-YYC-LHR.
Check individual segments vs. the entire journey. Some folks also use services like awardnexus.com where the website will do an automated search every day.