When and where were your parents born? When and where were you born? What gender are you (only relevant if your children were born abroad before 1983)? When and where were your children born and when and where were your grandchildren?
Broadly (and the answer may vary based on the specifics of your case), before 1983 British citizenship could be transmitted in the male line if the birth was registered with the local British diplomatic mission (something like what the Irish have nowadays). Since 1983, British citizenship can only be transmitted one generation outside the UK.
The main laws that you want to look at are the
Immigration Act 1971, which restricted the right to reside in the UK even for UK citizens who had no familial link going back two generations in the male line, and the
British Nationality Act 1981, which redefined British citizenship and restricted it to one generation (in exceptional circumstances two) outside the UK.
Also see this brief (out-of-date as it does not include the Immigration Acts 2014 & 2016)
history of the Immigration Acts,
of the history of British nationality and
British Nationality Acts through the ages on the Gov.UK website.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.