Although birth, marriage and death certificates can only be obtained by buying them from the General Register Office or from the relevant Local Register Office, there are ways of finding most of the same information from church registers of baptism, marriage and burials. Some of these church records are on pay-to-view websites like Ancestry and Find My Past. You may be able to view the actual church register or it may be a transcription.
There are also transcriptions of church registers available for free that have been compiled by volunteers. Eg
www.lan-opc.org.uk for Lancashire Online Parish Clerks (has links for other counties).
A very useful site is ukbmd.org.uk which has lots of links to useful databases, many free. Particularly useful are the indexes of birth, marriage and death certificates held by Local Register Offices, eg lancashirebmd.org.uk . Even if you cannot view the certificate online, you can use these to narrow down the search. Unlike the GRO indexes, these give more information, eg name of church where a couple married.