I think this is a stupid and mischievous idea, and personally I am disappointed that Guy has been allowed to bring it up yet again. In my opinion it is nothing short of trolling 
The simple fact is you won't get access to the 1921 census early. If you don't believe me look up the evidence given to Parliament by the ONS on their preparations for the 2011 census and the need not to jeopardise the creditability of censuses by breaking previously given undertakings
With regard to the evidence given by the ONS, it shows that people are -
"97. Concerns over
confidentiality of data and data-sharing within Government are at the forefront of the public consciousness in a manner perhaps not envisaged a decade ago. Londoners are far more aware of the capacity of Government bodies to share the data they hold on citizens between them. No matter how robust the procedural and legislative safeguards in place to prevent inappropriate and unauthorised data sharing, issues of trust and data security are bound to be raised during the run-up to the Census and on Census night. "
That shows the population is concerned not with the data being made public as many would have us believe but that the population is concerned about government departments sharing the information.
"98. We well understand the purpose of the Census, the independence from Government of the UK Statistics Authority and the Office for National Statistics and the scrupulous safeguards which are in place to prevent data sharing and the disclosure of personal data.
Others, for any number of reasons, may not: some, for example, may fear that accurate completion of a Census return will result in data being passed on to other central and local government departments and agencies, prompting reviews of benefit status or eligibility for council tax discounts. "
It is totally wrong to claim that the population is concerned about the public knowing their information. They are frightened of the government knowing their information.
"99. The purpose of the Census, and the use to which the information will be put, may need careful explanation to those resident in London who have fled repressive regimes or who are in communities with no tradition of engagement with the authorities to provide information of this nature. Colin Barrow, Leader of Westminster City Council, suggested that certain migrant groups may, because of their countries of origin, have an understandable inherent suspicion of providing information about themselves and their families to authorities:
"
Then there is fear of authority—a lot of people come here because they don't like where they've been, and it's authority that is the reason why they don't like where they've been; lack of cultural understanding—some countries have no tradition of Census, engaging with authorities and getting involved in all that, and I am briefed that Somalia is one such."[84]"
Again this displays the fact the population has a fear of governments not other individuals knowing the facts
It is such distortions of the truth that leads to data such as that supplied on census to be withheld from the public.
When the actual facts are disclosed the fears are entirely different from those some would like us to believe.
The census is put in jeopardy by people like Len Cook one time Registrar General making claim he was not authorised to make.
People see they have been mislead and then refuse to believe what authorities say in the future.
Cheers
Guy