Viv
Up till sometime after WW2 the firm was actually called Cadbury Bros., so the apostrophe did not apply in official documentation. But where the word Cadbury's was used, as far as I can see, it always had the apostrophe. Attached is a van from 1906 where the apostrophe is used on the side, and posters from 1853, and Cocoa Essence pictures from 1866, in all of which the apostrophe is used.
However, in early days sometimes just Cadbury was used , as can be seen on the side of the steam lorry in 1904 and the box of chocolates from 1866