In 1971 the British monetary system converted to decimals, and penny, pence and pounds are the coins available today. Before 1971, the penny, shilling and pence were the coins in use. The one-penny was a copper coin for years. A "copper" is a reference to an English penny coin, although the coin is now made of copper-plated steel, according to The DiCamillo Companion website. Standards of coin grading determine the value of a one-penny English coin, but expect to pay about £1.90 to £29 for the purchase price of a 1921 one penny.
Inspect the penny carefully. The 1921 one penny is a George V penny, and you can view a similar coin on the Professional Coin Grading Service website (see Resources). Be certain you have an English one penny coin, as there are similar coins that are not English. The coin should have "One Penny" on the backside.
Use a magnifying glass to check the condition of the coin. Compare with other coins shown on the photo-grade online resource to see what elements are important in grading the coin. Copper coins are graded by colour as well as condition. Study the grading system and decide if the coin is valuable enough for professional grading. Graded coins are more valuable than non-graded coins, but professional grading is expensive.
Purchase a 1921 one-penny coin based on condition, colour and sharpness of the image on both back and front. Value is dependent upon these factors, and a circulated and worn 1921 English coin is not valuable; an uncirculated or near-mint one penny coin is valuable. If you have a coin you believe may be valuable, be sure to encase it in a coin cover so it does not incur damage from oil on fingers and discolouration from the air.