I know, what I was saying is that if I could book it the other way around, surely mixing the two types should be allowed under the fare rules and that it's more likely an AC IT glitch.

Although actually right now it won't let me book PE lowest on LHR-YVR even if selecting PE lowest on YYZ-LHR.

So, pretty sure there's something wrong with AC.com's multi-city pricing on this one.