From here you can safely purchase a range of government-issued certificates, which can be used for any official purpose. Replacement certificates are often required for legal purposes, or required to be presented when moving abroad. They can also be used to track your family history.
Please keep in mind these are replacements for certificates issued abroad for British Citizens. If the birth, marriage or death of a British Citizen did not take place abroad, you must opt for one of the other replacement services instead.