You do not indicate why you think Sarah Laswell has a connection to Loudoun County, Virginia.
There were the following in Loudoun, from the Index to the 1758-1781 Loudoun Co Tithables as abstracted by the Sparacios:
Lasswell, Andrew - 1 tithable; 5 squirrel scalps, List taken for Shelburn Parish by Leven Powell 1771
Laswell, Andrew - 1 tithable, List taken by Leven Powell for 1775
Lasswell, Andrew - 1 tithable, List taken by Levin Powell, Shelburn Parish January 1777
Lasswell, Elias - 1 tithable, List taken by Levin Powell, Shelburn Parish January 1777
Laswell, Jacob, John Masson - 2 tithables, List taken by Richard Coleman for 1760
Lasswell, Jacob Moses - 1 tithable, William West's Tithable List - 1761
Lasswell, Jacob, John Mason - 2 tithables, William West's Tithable List - 1761
Lasswell, Moses - 1 tithable, List by Francis Peyton for 1758 for Cameron Parish
Lasswell, Moses - 1 tithable, List of Tiths. taken 1768 Leven Powell, Gent.
No Lasswells appear on the 1782-1830 Loudoun County Personal Property Tax Lists.