The 1921 census and later, can not be released for 100 years due to acts of Parliament.
On what basis was 100 years chosen? Did most of the population voice this opinion? I wonder what the opinion is now.
I should mention your first sentence is inaccurate.
There is no statutory 100 year rule for the any census; there was one imposed in 1966 by the Lord Chancellor's Instrument no. 12 of 1966 which was repealed in 2000 by the Freedom of information Act.
However this was debated in an adjournment debate in Parliament in 2004 where the question was put and agreed to.
http://www.rootschat.com/links/01n2h/ Having said that Parliament was in fact mislead earlier by being given inaccurate information about the legal position of the release of census information and indeed the Registrar General who made that claim, Arthur Roger Thatcher had to write to Parliament to apologise for exceeding his authority by adding a statement on the 1981 census schedules that it would not be released for 100 years a statement he had no authority to add.
The 1921 and later census come under the 1920 Census Act as amended by the Census (Confidentiality) Act 1991. This prohibits the release of information from the census without limitation.
I would also point out that when the 1911 census was taken most census were released after 50 to 80 years not 100 years. The 1931 England & Wales census was destroyed by fire due to enemy action but the 1931 Scottish census is in the archives waiting to be released.
The statistics from the various census up to 2011 are released after about a year or so but these do not include names so are of little use for genealogy.
The so called 100 year rule is non-statutory, that means it does not have the authority of law (statute).
In other words the present 100 year rule is simply a nice comfortable “office policy” rule unlike the previous 100 year rule that was introduced under a statutory power ( Statutory Instrument No. 12 of 1966).
The 1920 Census Act puts no time limit on the period of non-disclosure of personal information from the census.
http://www.rootschat.com/links/0wkf/The Census (Confidentiality) Act 1991, which amends the 1920 Census Act, makes it an offence for the Registrar General or any person under his/her control or who supplies services to them to disclose and personal census information.
http://www.legislation.gov.uk/ukpga/1991/6/introductionAgain no time limit is placed on the non-disclosure clause.
This in law means no personal information can be released from any census taken under the Census Act, 1920. Having said that there are legal ways round this one of which I attempted to use just after the release of the 1939 National Register but when I asked for support from family historians and from Family History Societies there was no support offered so I stopped my attempt to have the 1921 census released in 2010.
Further Information.
It has been recommended by the Advisory Council on Public Records and Archives that the closure period on census be 80 years.
In my view it is up to the government to set a closure period on census.
If they decide that the closure period should be 100 years then that would be acceptable though I would prefer 80 years.
It is possible that the Statistics Board (though unlikely without pressure) may release later census information early as the legislation they are going to use to open the 1921 census in 2022 allows them to open the 1921 census or any later census today or whenever they choose.
Sorry for such a long post, hope this helps.
Cheers
Guy