Some of the most valuable UK coins include early large-sized Guineas, oversized British gold coins and coins minted before Queen Victoria's reign. Most of these coins are made using large amounts of gold and may not be dated. The imprint of the reigning monarch is often used to determine age.
The large-sized Guineas are valuable because of their gold content. In the time of King George III, which was 1760 to 1820, these coins were similar in size to a U.S. half dollar and as of 2015, worth about $1,000. Half-guineas, one third-guineas and quarter-guineas were minted later but are still valuable for their gold content and historic designs.
In the early 1800s, some of Great Britain's coins contained over a Troy ounce of gold. Known as 5-pound coins and sometimes labeled Massive British Coins in the trade, they could be worth up to $10,000.
Queen Victoria reigned from 1837 to 1901. Gold coins printed before that time, which are called sovereigns or half-sovereigns, are very collectible. They carry the picture of the reigning monarch on one side and the British Coat of Arms on the other. Age and gold content determine the value. The first was issued in 1489 by King Henry VII.