Not sure if it's been mentioned, and not that is would make being imparied acceptable, but the pilot was a relief pilot, not the Captain.
It is my understanding that an Air Canada Staff physician did a blood test within the required 4 hours, and the pilot has been cleared, and positioned home through YYZ. Don't expect to read that in the news. I also wouldn't expect to read that according to a source at AC, they have been told the breath test at LHR has a 7% false positive rate.