Not to make excuses but your story probably provoked concern that you could have been doing a drug deal. The fact that you weren't subjected to secondary inspection indicates that your version was accepted. The border agents have expectations of what constitutes a normal visit and a day trip by train to London, as you found out, is not a normal tourist trip.
While I appreciate and understand your comment, if you could see us two 57 year old doofuses and could have heard his tone of voice, I think you would have concluded that this was more than just a legitimate concern that we might be running drugs. He was exceedingly rude and disgusted with our very existence( maybe he just got moved to the am shift and wasn't a morning person!) Anyway, it was no big deal since we made our train and had our little day trip. It certainly hasn't deterred me from coming back to Canada( but maybe for more than a day the next time)
ETM