BSI is a private company incorporated by Royal Charter. The charter was first awarded (to the British Engineering Standards Association) in 1929, and amended several times subsequently.
The Royal Charter is essentially an enabling document that sets out the purpose of BSI and defines in broad terms its range of activities, including its functions as a standards body, as well as its ability to offer training, testing and certification services. It does not, to any great extent, impose specific duties or responsibilities on the company.
The Royal Charter is supplemented by the Bye-laws which detail some aspects of how BSI has to be run. They cover matters such as membership, the conduct of General Meetings, and the composition of the Board.