I think it would end up depending on who the English person was, whether they were learned, . I know that as early as Plato, the great thinkers were arguing against the idea of dragons.

Doing some research into reference books, the 1771 edition of the Encyclopedia Britannica references real creatures under the draco entry but says no more.

In the 1823 edition on page 289, the entry has been expanded to explicitly shut down the mythological interpretation.

Obviously the date could be much earlier than 1771, but these are the references I was able to find.