economy (lowest price) vs economy (flexible)

I just did a quick search on aircanada.com for fares SYD-LHR in August, and the lowest price economy fares display at around $3,300 and the flexible economy fares at around $10,700. That seems like one hell of a premium to get you into the position to use a SWU.

Is that normal? Are there other sites where I could price out a specific Y, M, U fare for anything even moderately close to 'lowest price'?

You are facing a couple of problems. One is that you are booking somewhat late for the Canada-UK portion - it is high season, so you might be pushed into a higher fare category. Second is that I don't think AC's pricing on non-Canada to non-Canada destintaions is that great - it tends to "break" in Canada (SYD-Canada, then Canada-UK). You are being offerred all Y fares and "refundable" generally includes Y/M/U - the later two noticeably cheaper than the first. Also, consider routing to YVR and then YVR-LHR - cost (before fees, etc...) is much cheaper - 3,400 to YVR and then 2,200 to/from LHR.

Try kayak or another travel website that allows you to select "refundable" economy and see what you get. I've had better luck finding "odd" itins and booking classes where ACs website doesn't accept it.

Yeah just break into two - but I'd just call AC for help since there's no charge for YMU (Lat) fares... and then you can stop in YVR if you want too!

You can also use http://beta.itasoftware.com/ and search for

SYD:: O:AC+ / F BC=U
LHR:: O:AC+ / F BC=U

Unfortunately I don't know how to search for trips which combine different legs in different booking classes. I don't find any matches for booking class U or M and the fares for booking class Y show up over $10k.