How many seasons has Silverstone Circuit hosted the United Kindom grand prix?
British Grand Prix is the name of race; British refers to the United Kindom
SELECT COUNT(T2.circuitid) FROM circuits AS T1 INNER JOIN races AS T2 ON T2.circuitID = T1.circuitId WHERE T1.name = 'Silverstone Circuit' AND T2.name = 'British Grand Prix'