The supposedly ‘private’ data is to be shared with all 27 EU member states and any/all private companies that those governments wish to share it with, as seen here and this site explains all about it and breaks down the reasons the government gives for what is a blatant breach of privacy laws and human rights legislation.
The census is nothing more than a method by which the government can gleam the information required in order to raise taxes further and cut benefits / public spending. They have the majority of this information on file already from the electoral roll, National Insurance database, DVLA etc. and they are just lazy funkers than can’t be bothered to collate it themselves.
Just cross out your address and write “No Contract – Return to sender” on it and post it back to them. I for one have not signed any contract with Lockheed Martin, or the government agreeing to provide them with my personal information, so they can p right off!