Penny - pence - pennies
The word penny (a small coin) has two plural forms. One, pennies, is regular. The other is not – pence. As a rule of thumb, use the latter when you are giving a precise cost. When you are talking about the coins in general, use pennies – e.g. "I have a pocket full of pennies" but "The postage is twenty pence."
The position is sometimes complicated by the fact that those who learnt to handle money before the change to decimal coinage (see Pounds, shillings and pence) often use a form of plural belonging to the old system. This is the word penny added as a suffix to various numbers. A coin valued at just over 1p in modern terms was called a threepenny, pronounced (and sometimes written) as 'thruppenny' (in the North of England). The price was usually pronounced 'thruppence’. We also had things, be they ice creams, bus rides or cinema tickets called tuppenny (more formally twopenny), and in derogatory mood we could call something tuppenny-halfpenny, meaning 'very cheap'. The halfpenny itself was sometimes written ha'penny.
In slang, if a man gave another a fourpenny one, it meant 'hit him very hard'. There has not been a fourpenny coin for a long time. In the olden days, there was one, called a groat.
You may also want to see Pounds, shillings and pence, which outlines the traditional British currency.