If you're on a single ticket MAN-LHR-YVR, you should be able to check in for both flights online 24 hours before your MAN-LHR, if you are an Executive Club member. Some people report that this gets brilliant seats. My experience is that sometimes you do, and sometimes the full seating chart hasn't been released at -24 hours.
You probably won't get exit row seats online, because they usually want to see you in person before allocating them. They are worth asking for. But there is a school of thought that if you ask BA for an upgrade you diminish your chances of getting one.