This is not a cabotage violation. It is on multiple tickets and therefore perfectly saleable.
However, it is extremely risky. If there is a delay on your SEA-YVR and it causes you to no show for your YVR-LHR, that ticket will be cancelled and retain whatever value it retains under its fare rules. That may well be $0 and you will wind up having to purchase a new ticket.
In this situation, I would fly into YVR the night before.