This Is My Georgius1111 Dg Britanniar Rex F D Coin 1821

This is a crown (5 shilling piece).

in 1816, the British had "The Great Recoinage".
In 1817, the 5-shilling silver Crown, and the 20-shilling gold sovereign, when into circulation,
Both featured Benedetto PISTRUCCI's sculpture of St George & the Dragon.
Upto & including 1820, that image was surrounded by a "Garter", with a legend in Old French.
King George III died in 1820.
The only coin featuring the new King in 1820 was the half-crown.
In 1821, the full range of denominations were issued, featuring KG4, but the Garter was omitted from the designs.
So, your coin was the first year of that new design.

Co-incidentally, or by design, St George was the patron saint of England.
King George was the King of England, and later of the United Kingdom.
I have heard it asserted that St George was a metaphor for England, &/or the King;
and the dragon was a metaphor for Napoleon;
and the whole image represents the end of the Napoleonic Wars - successfully, from the English perspective.
And, it's a beautiful piece of art.