Hi Alison
Family search is the site of the Latter Day Saints (Mormons) who have transcribed many church registers and made them available to the public. There are also records on there that have been submitted by private individuals (like you or me).
It is not a complete record of anything. There is no particular pattern - some records have been transcribed and others haven't. It is feasible to reason that there won't be items on there for people who are still alive. Whilst the IGI baptism and marriage records can be very useful for locating a family, you should always remember that there are many many more that aren't on there. Also, it is only really a finding aid/index so you should always check with the original records where you can.
If your ancestor was married in a register office, they wouldn't be on there anyway as the only way that you can get a copy of such marriages is to purchase a certificate.
Can you be sure that these ancestors that you have found back to the 1700s are the correct ones.
The complete records for births, marriages and deaths for England is at the General Register Office (1837- today). These are gradually being transcribed by volunteers at
http://freebmd.rootsweb.com/You can view the information pages to see the progress of the index.
There is a complete set of the indexes at
http://www.1837online.com/Trace2web/where you have to pay to search them.
If you know the area where you ancestors lived, then it might be feasible to search the baptism and burial registers of the actual church yourself if you can.
The 1881 census is available on the family search site but it is nothing to do with the index of baptisms and marriages etc.
Just start with the information that you are absolutely certain about - yourself/parents/grandparents? and work back one step at a time.
Hope this helps
Casalguidi