Counterfeiting - the act of imitating or forging something trusted is a serious crime. Throughout history debasing the coin of the realm by counterfeiting has been considered one of the most serious offences against the state, often punishable by death. Nonetheless, counterfeits of higher-value coins designed for general circulation at face value have been made by criminals for thousands of years.
Modern counterfeit coins are not generally made to a very high standard so that they cannot easily deceive experts. But passing off false currency in a transaction at the shops is much easier, hence low-value counterfeit coins continue to circulate. For modern coinage the most common method of protection from forgeries is the use of two metals of different colours which make them difficult to counterfeit at low cost. The most common way of forging these coins is to change the area that should be a different colour by painting it, however the coins soon look very crude once worn.
Bear in mind that relative to the amount of currency circulating in the UK counterfeiting is very rare.