Great Britain was the first country to adopt a truly modern postal system and, as a result, has never been required to stamp its country name on postage. Prior to the adoption of the Penny Black, the first adhesive postage stamp, most postal systems collected fees from the recipients of mail, according to Stamp-Collecting-World. The British system was the first to adopt prepayment, the proof of which was the stamp.
For 125 years, nearly all British postage bore the portrait of the reigning monarch. In 1965, however, British Postmaster General Tony Benn developed new criteria for British stamp design, according to Wikipedia. The new rules permitted the minting of postage stamps that did not prominently feature the monarch, but still retained some elements of the traditional royal focus. The compromise that was reached was to use a small silhouette of the monarch, Elizabeth II in this case, in place of a country name. This only applies to commemorative stamps. The silhouette serves as an alternative identification method for British postage. The Queen's silhouette has been in continuous use since its adoption for most stamps that do not bear the full portrait of the Queen, Prince of Wales or other royal figure.