The British Imperial system of units was established on June 17, 1824, by the Weights and Measures Act. Strictly speaking, it is not correct to use the term British Imperial for units in use before that date, and they are better described as traditional British. The British Imperial system was based on three units, the pound avoirdupois, the yard, and the second.
In 1963, a new Weights and Measures Act defined the yard and pound avoirdupois in terms of the metric units, conforming to an agreement with the United States, so that the values have since then agreed with the International yard and International pound avoirdupois. Details of these changes will be found on the pages describing the British Imperial system of units of length and the British Imperial system of units of weight