Although the Pears' Soap underprints do not fall under the same umbrella as the protective overprints and underprints, as they were essentially  for advertising rather than security purposes, I have included them here for completeness.

A. F. Pears Ltd. the soap manufacturer,  used the above picture (BUBBLES by Sir John Millais) as their main advertising symbol.  The above image is taken from a postcard of the 1930's.  The reverse of the card advertises the fact that the original painting, purchased by A. & F. Pears. Ltd., was to be on display for a period of time in a Leeds store.

During the reign of Queen Victoria a number of British stamps were produced with an advertisement for the firm in the form of an underprint.  These underprints appeared on the 1881 1d lilac with 16 dots (S.G. 173) and the 1887 1/2d vermilion (S.G. 197) shown below: