Unfortunately, no one submitted the correct answer. Which either means no one is paying attention or my trivia questions are getting better.
Following The Great Post Office Reform in 1839, after trying a number of alternatives, Britain moved to a postal system using fixed rates for sending letters. To facilitate proof of payment of the postage, an adhesive label was added to the letter. By 1840, the Penny Black, the first ever postal stamp was issued. At the time, since stamps were only for national postage, there was no reason to put the nation’s name on the stamp. Afterwards, when cooperation and formal treaties arose with foreign postal services, the British ‘no name’ design was respected, but that was agreed only if British stamps included the current British royal regent in the design – which remains the agreement to this day.
As an interesting side note, there have been times Britain did choose to include the word “Britain” in a stamp design, the first being a special issue commemorating the Festival of Britain in 1951.