If you are on BA in D, I much prefer the LHR - (SFO/LAX/YVR/SEA) flights as you have a better chance of getting some good sleep if you pick times correctly. The trans Atlantic run to the East coast of Nth America is a pain as there isn't really enought time to do anything.
As for maximising miles, the beauty of the xONEx fare is that you can back-track within a continent. That opens up many options for very inefficient travel.