In 1974 county boundaries were changed and Slough, previously in Buckinghamshire, became a borough within the county of Berkshire, England. In 1998 Berkshire organized its local government structure into unitary authorities, one of which was Slough Borough.
The transfer of Slough from Buckinghamshire to Berkshire is discussed in greater detail on the page on the town of Slough.
For more information, see the EN Wikipedia article Slough Borough Council.
For more information, see the EN Wikipedia article Slough.