Family Search, run by the Mormons is free. Other websites that have a free trial period and then charge include My Heritage and FindMyPast.
There were censuses in England and Wales in 1841, 1851, 1861, 1871, 1881, 1891, 1901, 1911, and 1939 free on Family Search. 1921 is afaik only on FindMyPast.
It helps if you know the birthdate approximately for one of your grandparents or great grandparents. Say your grandfather was born in 1900 and your grandmother in 1899, you can search them in the 1901 and 1911 census. It helps if you know where they would have living - just the town or county. As they would have been children, you can find out who was head of the household, their wife, all the children, any lodgers (possibly members of the wider family) and servants living in the specific address on that day.
Alternatively, you can search the birth, christening, marriage, death and burial records for England and Wales - civil registrations or parish records. It helps if you know the mother’s maiden name, as that was often put on the birth certificate. For instance, if the father was John Wallace and the mother’s maiden name was Janet Wheelwright, living in Aylesbury, Buckinghamshire, you can do a search on father’s surname Wallace and mother’s maiden name Wheelwright in Aylesbury in the birth records for a 2/5/10 year period say around 1900. Sometimes a child’s birth record just has their name, the quarter and registration district, but you should be able to find them in the next census.
What these websites don’t tell you is, who the biological parents were. Families of 8, 10, 12 children seem to have been common. Say your grandmother took in a baby niece, whose parent had died and gave it her husband’s surname in the next census, its hard tell if its her’s or one of her 10 siblings’ 10 children…Its a lot of research to look at 100 other children! People either lied or told mistruths on the census, probably guessing nobody was going to check before computers.
For instance, according to the marriage records, my great grandparents had 4 children before they got married. Three were given their father’s surname and one was given their mother’s maiden name on the birth certificates. On the next census, my great grandfather gave all of them his surname and wrote he and my great grandmother had been married 9 years - when the marriage records show they’d been married 2 years! Why did one girl officially have her mother’s maiden name, and the other three born out of wedlock didn’t?
There are many other records available such as military, immigration, Scotland, Ireland, and the US, if you need them.
I haven’t looked at My Ancestry, which iirc, has family DNA on it, and I don’t know how much they charge. Many children died young in those days.