The tariff never specified English goods.
The McKinley Tariff was originally passed to protect the American farmer. If you read his original speech given in Congress that is obvious. It never achieved this goal. It did set off a series of "tariff wars" that raged in Congress for almost 40 years. These varied in their impact on the trans-Atlantic gun trade. Completed guns suffered the most.
It is interesting to note that the .410 was developed in England, but did not appear here until 1915. Perhaps a direct result of Woodrow Wilson's election and his efforts to take the tariff's apart.
On the flip side, because European firearms where artifically more expensive, many small firearms companies were created here. Not the least of which are names like Lefever, Fox, Parker, LC Smith, etc. Many of whom were able to import barrels and other parts of European manufacture because the tariff treated them more kindly than an assembled firearm. During this period we have examples of European firms licensing American patents and producting the guns for their market, but few examples of the reverse.
Pete