A national population census has been taken every ten years since 1801. The earliest still available is that for 1841, the others having been destroyed due to containing only statistical information. The only exception to this ten year rule is that there was no census taken in 1941 due to WWII. Census returns are not open to public scrutiny until 100 years after they were taken, although indications are that this rule will be relaxed slightly due to public demand. The next census, that for 1911, is now available online
Below you will find links to all of the UK Census returns currently available online.