Decimal Day in the UK was in 1971. That's when the value of a British penny became worth 1/100th of a pound. Prior to that, a penny was worth 1/240th of a pound. To make the switch, the Royal Mint issued coins with the NEW PENCE denomination. They continued for 10 years. After 1981 the NEW was dropped and the Queen's portrait changed. Pence is the plural form of penny.
These coins are made of copper-nickel. Normal circulated coins are worth 50 pence in the UK. At current exchange rates (see xe.com for the current rate), that is about 80 US cents.
Sometimes coin collectors buy nice-looking examples of modern coins to add to their collections. If you have a 50 pence in fully uncirculated condition, you can probably sell it to a collector for a few US dollars.
That is Britannia, a symbol of strength and virtue, on the back of these 50 pence coins.