Today, I will attempt to give a synopsis of why parish records may have been lost, and also where you may be able to locate those that have survived. For some details, I am indebted to the book Tracing Your Georgian Ancestors 1714-1837 by John Wintrip. I did say I would explain where you might find the parish records, but I feel it is important for me to explain more about the records first.
With a few exceptions, mostly in urban areas like London, separate baptism, marriage 
and burial registers were not kept. That is, until 1754, when Hardwicke’s Marriage Act required separate marriage registers to be kept. Most parishes listed the various events separately, meaning that all the baptisms on a page were in the same place. But if you are unlucky, you will find baptisms, marriages and burials all interspersed on the same page.
Because only minimal information was included, people sometimes had difficulty proving their identity or relationship to another person. 
An example would be trying to establish a right to inherit land or property through a will or intestacy. Perhaps in part prompted by this, Ralph Bigland wrote a book in which he made recommendations for more genealogical information to be included in parish registers. No rules were changed at that time, but in quite a few parishes, the entries in the registers did start to include more information. Some bishops even recommended that course of action. Even so, this improvement was spotty, to say the least, and it would not be until 1813, just 24 years before civil registration began, that rules were introduced requiring this.
George Rose MP introduced a comprehensive bill into Parliament in 1812 which would 
have required genealogical information for all baptisms, marriages and burials. Much to his disappointment, the rules were considerably watered down during the Bill’s passage through Parliament. It is ironic, therefore, that the Act is commonly called Rose’s Act. Even so, from 1813, parishes were required to keep separate registers for baptisms, marriages and burials, and to enter the information in printed forms in a bound volume to be kept in an iron parish chest. Many parishes, though, continued to use the old wooden chests.
By no means all parish records are available. Reasons include simple neglect by those having care of the records, destruction of the churches, especially during air raids in the second world war, fire, and flood. Records may also be missing between 1783 and 1794 because of a stamp duty imposed on every baptism, marriage, and burial entry in the register.
And next time I will talk about how to access the records.
Family History research in England 