user: Question: What was the fastest lap time at the British Grand Prix with Mercedes as the constructor?
Columns: ['Race', 'Circuit', 'Date', 'Pole position', 'Fastest lap', 'Winning driver', 'Constructor', 'Tyre', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Argentine Grand Prix', 'Oscar Gálvez', '16 January', 'José Froilán González', 'Juan Manuel Fangio', 'Juan Manuel Fangio', 'Mercedes', 'C', 'Report']
SQL:

assistant: SELECT "Fastest lap" FROM "Table" WHERE "Constructor" = 'Mercedes' AND "Race" = 'British Grand Prix';