"Cheerio" is indeed a word that goes back a long time, especially in jolly old England. But "Cheerios," with the "s" at the end, is a federally registered trademark owned by General Mills, and GM might not want to have their brand associated with "bloody."
"Cheerio" as a word could likely never be trademarked, because it is a word in common useage, although it potentially could be trademarked as a logo. But "cheerios" with the "s" at the end appears to be a made-up word, and made-up words are generally entitled to strong protection under trademark laws.
