In answer to your main question, the relevant legislation did make it an offence wilfully to provide false information on a census return (among other things). See for example s12(2) paragraph 2 of the Census (Great Britain) Act 1910 which enabled the 1911 census to be taken:
12. [...]
(2) If any occupier for whom a schedule is left under this Act—
1.Wilfully refuses, or without lawful excuse neglects, to fill up or cause to be filled up the schedule to the best of his knowledge and belief, or to sign and deliver it as by this Act required; or
2.Wilfully makes, signs, or delivers, or causes to be made, signed, or delivered, any false return of any matter specified in the schedule; or
3.Refuses to answer, or wilfully gives a false answer to, any question necessary for obtaining the information required to be obtained under this Act;
He shall for each offence be liable on conviction under the Summary Jurisdiction Acts to a fine not exceeding five pounds.
However, the reality is that there was little or no policing of the veracity of the information returned, and family historians tend to find that all the published censuses are riddled with lies, half-truths and omissions. Enumerators had a tough enough job getting all the forms delivered, collected and transcribed without doing amateur detective work on the contents.