Royal Mail have a monopoly on letter delivery in UK, but in 1980 they realised that at Christmas periods they were always overwhelmed, and decided to waive their monopoly. Under the terms of a General Licence, they allowed charities to undertake local delivery of cards and letters for a fee, during the Christmas period.
When this happened, the Boys' Brigade in the UK issued sheets of stamps for BB Companies who wanted to take advantage of the relaxation of the monopoly. The stamps were intended to make it easier for BB Companies to use as a means of fundraising.
Unfortunately, the uptake was very limited, leaving the BB with many sheets of surplus stamps. They therefore overprinted them with the silhouette of Sir William Smith - and you can still read the 1980 date beneath that outline (top right-hand corner) if you look closely! Eventually surplus stocks of the stamps were destroyed.