Welcome to Flyertalk
feist, welcome in particular to the BA board, and I hope your trip to London goes well. Given that poor old Calgary used to get just about the worst selection of BA's aircraft in the past, a situation that seems to have persisted for very many years - the switch to the 787-8 will make a big improvement to your experience. Provided you are not in World Traveller.
I would say 3A and 3B are good options. For anyone sensitive to light then also 3D and 3E would be good, 3E being very dark. It also has direct aisle access and a bit more space than other seats. The light issue probably won't impact many people, since though a lot of people need darkness to get to sleep, the presence of half light doesn't necessarily wake them up. The flush mechanism on 787 toilets is very quiet, less than half the noise of an older jet I would say.
I think it's also fair to say that if you are a couple then on the 787 aircraft there aren't the "go to" seats that there are on wider aircraft with larger cabins. The differences between seats aren't so great, though the taller passenger still benefits from those where the footstall is against a bulkhead. So if light and noise sensitivity is an issue
On a Calgary to London service I don't think the sun comes up on the port (A) side of the aircraft, though with all the enhancements we're having at the moment I guess we can't take anything for granted. Furthermore this website agrees with me:
http://sunflight.net/
The sun will be more or less in front, starting to appear about 5 hours into the flight, over Greenland with a bias to the starboard side until the last hour or two when the light will be stronger and more emphatically starboard.