I read an article about a similar situation in which a UA-ticketed customer was flying on US and AC metal, and when he went to check something, found out that US didn't have a record of his booking. It got sorted out, of course, but there seems to be some inability among the three North American carriers to perform some non-standard code-share booking transactions automatically. I wouldn't be concerned in the least unless you are really, really set on the exact flights booked, and even then, I would probably wait it out. Worse case scenario, you leave SFO an hour later on AC metal and get to LHR roughly an hour later because AC has five flights a day from Toronto, four of them in the evening.