They pronounced "Boeuf" as "Bully" or "Billy"? - that would surely have meant some very funny printing.

But the expression "bully beef" meaning tinned corned beef is still pretty common in England anyway. Brewers Dictionary of Phrase and fable suggest, convincingly, that this comes from the French word "bouilli", meaning boiled. So if that was printed on the tins, it would make the explanation more convincing. And it would imply that the billy can came before the Billy Tea, rather than the other way round.