TNA have announced that there will be free access to the 1921 census at Manchester Central Library and the National Library of Wales as well as at TNA itself:
https://www.nationalarchives.gov.uk/...S3X9d6oXwx_Pts
It doesn't say whether this access will be unlimited or capped, though. They did something similar with the 1911 census at various county archives but they were restricted to a certain number of "credits" which were used up much more quickly than expected and then it was cut off.