This site has links to free websites with church parish registers from the United Kingdom and Ireland. For example, a search on England, Norfolk County, and the parish of Upwell produces links to several sites, including an index to baptisms, marriages and burials on the FreeReg website.
A free-to-search volunteer project to transcribe the English and Welsh civil registration indexes, FreeBMD covers nearly 266 million records. All years are not yet available, but more transcriptions are added regularly.
This terrific site provides free access to transcribed church baptism, marriage and burial records from parish registers, Nonconformist records and more. Operated by volunteers and regularly updated, FreeReg has more than 40 million records and should be one of the first websites you check for UK ancestors before 1837.