Although the nominal sizes are similarly labeled the main difference lies in the thread profile and resultant relationship of the pitch diameters. Both systems identify their nominal diameters primarily by factional inches however; the British use a 55 degree thread angle with radiused root and crest. The customary American profile is 60 degrees with angular root and crest. Even though you may find a diameter and pitch combination that may seem to match it should not be used. The resultant assembly will be significantly weaker since there will be excess play due to the different angles and the threads will be binding on the root and crest rather than the pitch diameters.