http://www.nisranew.nisra.gov.uk/census/pdf/FACT7.PDF
The Census is compulsory
Every household must return a form by law. This is because it is
important for people not to be missed out – the Census is designed to be
a complete count of the population. Failure to make a completed return,
or giving false information, is an offence which may attract a fine of up
to £1,000
I have never objected to supplying info in the past - now I have serious doubts about how it will be used.