British coins reflect a nation with a very long history and traditions that take a long time to die. Using Latin on coins is not anachronistic but a statement of who we, the British, are and hopefully will always be, reinforced by the effigy of Queen or King on the obverse and emblems of greatness, like Britannia, on the reverse.
History does take its toll; Over the years we have dropped a few phrases. Britain no longer has an empire, colonies or is even a major influence in many countries. We rarely say 'Great' Britain. These days Britain is more often referred to as the 'United Kingdom'. But coins give us, at the very least, an artistic license to maintain a practice dating back more than 500 years.
Some Latin phrases are still used on coins associated with Britain or the British Commonwealth, such as Canada.
What does the Latin on a modern British coin mean?
Modern coins are smaller in diameter than those of the pre-decimal era, a sign of the times that even non-precious metal is expensive, so the first thing to note is that the inscription is Latin abbreviations .
On the obverse ('heads') the portrait of the Queen is surrounded by the text (called the Inscription or Legend):
ELIZABETH II: DG REG: FD
So we have:
The British Monarch, which is currently Queen Elizabeth the Second. Many monarchs use the Latin spelling of their names but not in all cases.
An Abbreviation of DEI GRATIA REGINA, which means 'By the Grace of God, Queen'. (A king would have DG REX).
An abbreviation of FIDEI DEFENSOR, which means 'Defender of the Faith'. The British Monarch is also head (Supreme Governor) of the Church of England. In the Church hierarchy, the Monarch is above the Archbishop of Canterbury. This subsidiary title was given to Henry VIII by Pope Leo X and the phrase has been been used on coins since 1714 with the exception of a Florin in 1849 which was known as the 'Godless' Florin.
Many Commonwealth countries have dropped this phrase due to its association with the Christian Church. Prince Charles (who is the current heir to the Throne) has been reported as saying he would like the term to be 'Defender of Faiths' rather than 'Defender of the Faith'. Another example of how inscriptions evolve in a changing World.
Prior to decimalisation, coins tended to have a much bigger diameter and so despite the phrases were as above they were not always abbreviated so much. As noted above, a few other phrases were used which have now lost favor. Consider a pre-war Penny of the 1930's, it would read:
GEORGIVS VI: DG BR OMN REX FD IND IMP
(In passing you may have noticed that successive Monarchs face in opposite directions. This is just a tradition).
The Monarch then was King George the sixth. Note the spelling of the name is written in Latin. Some are more confusing, for example Carolvs II is Charles the second, Iacobvs II is James the second.
Dei Gratia as before. All Monarchs rule with God's Grace. Some coins use DEI GR or DEI GRA.
BR or BRITT refers to Britain.
Omni means 'All' or 'Every'. In context of the coin inscription it's promoting the Monarch as Ruler of his subjects, 'King of All Britains'.
Rex means King (as Regina means Queen)
As before, FIDEI DEFENSOR, 'Defender of the Faith'.
IND is for India and IMP is Emperor, so we have 'Emperor of India'. India became an independent nation in 1947 and the phrase was discontinued.
There are probably a few other Latin words, but these are the most popular.