I went into my local library the other day to ask whether there was any access to the 1921 census, and got talking to the staff member about Ancestry. She said that the version they use isn't the full version that you can personally subscribe to, and you can't access all the information.
Has anyone used both? I'd be interested to know what the differences are, as I'm thinking of doing some more research soon and don't know whether to rely on the library one, or whether it will be too limited.