Because Britain is the United Kingdom, which consists of England, Scotland, Wales and Northern Ireland. His national flag is a superposition of several flags.
Look at the origin of the British flag:
1277, the British designed a "St. George's Flag" with a red cross on a white background as the national flag of England.
1603, King James I of Scotland also served as King of England, unifying England and Scotland. James thought that there should be a flag representing England and Scotland, so in 1606, the Scottish flag, the "St. Andrew" with a blue background and a white X and the St. George's flag merged, and the earliest rice flag appeared. But not the British flag we see now.
1648, the Puritan Revolution took place. Cromwell, the leader of the Puritans, executed King Charles I of England, abolished the royal government and the House of Lords, established the * * * regime in 1649, and changed the national flag to the national flag of England. 1660, the royal government was restored and the national flag was changed back to Miqi.
180 1 year 1 month 1 day, Ireland's red X-shaped "St. Patrick" was incorporated into the British flag, becoming the current style, which has been used ever since.
So now the British flag only appears in 180 1. The Danish flag is the oldest.