This is an incredibly broad question with no real answer. Since British Coins have been issued for well over a thousand years, there are numerous rare and expensive coins and really no data to show what is the most expensive of the old British coins.
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If you want to buy old British Pennies, check your local coin dealers. Most coin dealers will have stocks of circulated 20th century Pennies for a few pence each. If you are looking for something a little more upmarket, coin dealers will be able to help, or you could try eBay. You only get what you pay for.
Old Kent Road
In Indianapolis, old British coins are typically purchased by coin collectors, numismatists, and antique dealers who specialize in currency. Local coin shops, online marketplaces, and auction houses also attract buyers interested in rare or historical coins. Additionally, collectors may participate in coin shows or clubs where they can find others interested in trading or purchasing old British currency.
Most coin dealers will do this for you.
The Modern one pence piece and the old three pence piece
A Guinea is a coin of the old pre-decimal British currency. It was worth approximately 21 shillings.
Three pence piece (thri'pence) abbrieviated as 3d, a British coin made originally in silver and later in brass.
For a coin to be considered to be an "antique" it must be 100 years old or older just like any other antique. Though most coin collectors never will refer to a coin as antique.
If you refer to a British coin, and depending on the age of the coin and where the characters are located on the coin, it could have one of two purposes. It may refer to the regnal year of the monarch, most likely on a silver Crown coin. XII being the twelfth year of the reign of King Bob III or Queen Susie VI for example and may appear on the edge of the coin. Alternatively, on very much older coins, it may refer to the face value of the coin and appear on either the obverse of the reverse of the coin. XII will most likely refer to 12 Pence, making the coin a One Shilling coin. In this case, the characters are likely to be much larger than other characters on the same side of the coin. Since no British Monarch has numbered higher than VIII yet, on a non-British coin it might refer to which monarch for a common name, for example the French King Louis XII.
The Florin (Two Shillings) was a coin of the old redundant British predecimal coinage system. The coin may have come from any of the 50 plus British Empire/Commonwealth countries. If it is a British coin, there will be no country name on the coin, otherwise the coin will indicate the country of origin.
No. A farthing was a small British copper or bronze coin valued at one quarter of a Penny. A Shilling was a silver coin equal to 12 Pence.