Abi,
(I presume that's your name and that you are not from the Association of British Insurers!)
I work in the insurance industry. I am not aware of any legislation that requires building or contents insurance. It is widely perceived as a wise move for anyone who can't afford to buy another building with the spare cash they have, but not required in law. Banks, if lending money to buy a building, will often require insurance. Shareholders would often expect their assets to be protected by insurance, but if you were a massive, cash rich organisation, who's buildings were well spread (not likely to be damaged in the same fire/flood/explosion/plane crash) who managed their risks excellently, you may "self-insure" or even no insure your buildings. This is very rare however.