You need to check your fare rules (which is probably difficult now, since I doubt you have them.) I fares sold outside the UK are usually more flexible but I don't believe you can apply the remaining value to a new booking. Ex-UK I fares are completely restricted, though POS elsewhere have different change conditions. I don't believe refunding is one of them, but you might be best off calling BA.

Will you be travelling to YVR anytime in say the next 12 months? You could just change the dates (and then be faced with a one-way for the return.)