AutoCAD 2012 Mechanical does have BS standards I believe Mechanical 2008 did as well. They should be found in the Steel Shapes dialog in the Standard Parts toolbar blended in with the rest of the standards that were picked during installation. If you do not have them installed, run the "Uninstall or change a program" wizard and modify the installation. Check the customize installation and there is a page to select which standards you would like installed. You may be prompted for the install disk.

For anybody looking for shapes that does not have Mechanical, you can find them here. I am in the process of exporting the entire shapes library and placing the content online.