Coat of arms of the United Kingdom depicts harp
History
England and Scotland
The current royal arms originated in the arms of the Kingdom of England and Kingdom of Scotland, both adopted in the twelfth century. The English arms were quartered with those of France from 1340 (except 1360-69), representing the English claim to the French throne. The arms of Scotland remained unaltered except during the reign of Mary, Queen of Scots, when they were first impaled with those of her husband, Francis II of France, and then quartered to represent Mary's claim to the English throne. Similarly, during the reign of Mary I of England her arms were impaled with those of her husband, Philip II of Spain. 
In 1603 James VI of Scotland inherited the English and Irish thrones, and to symbolise this union of the crowns the arms of England (including France) and Ireland were quartered with those of Scotland. In 1689 Mary II and William III became co-monarchs and impaled their arms; both used the royal arms, with William also bearing an inescucheon of Nassau, the royal house to which he belonged. 
During the Commonwealth and The Protectorate in the mid-seventeenth century the arms were significantly changed, as the monarchy had been abolished. The Irish harp continued to be used, but England was represented by St George's Cross and Scotland by St Andrew's Cross. These were impaled in various ways, and from 1655 to 1659 also included the arms of the Lord Protector, Oliver Cromwell, on an inescutcheon.