There is a requirement on Canadian registered aircraft for the crew to be fluent in French and all announcements (even between UK and the Med) to be in both English and French. It is likely therefore the easiest way for that to be proven as complied with, is to put a SWG crew member on board.
Some of the aircraft going to Canada sometimes remain on the UK register, some are re-registered onto the C register. The SWG crew can therefore comply with any CAA rules since they all speak English.