On Chabad, 1826
The article, from the October 1834 issue, is called The Karaites, and other Jewish sects and contains material from a book called Biblical Researches and Travels in Russia (London, 1826) by Ebenezer Henderson.
He informs the reader that
"The most popular sect among the Jews, is that known by the name of Rabbinists, or Talmudists, i.e. such as yielf implicit obedience to the doctrines and institutions of the Rabbins, as delivered in, or deducible from the Talmud, and who, according to the general acceptation of the term, may be accounted the orthodox....They are precisely, in the present day, what the Pharisees were in the time of our Lord....But although the Rabbinists compose the great body of Jews in Poland, there exist other denominations, the numbers and pecularities of which are too considerable not to strike the inqisitive traveller.After describing a version of the history of Chasiddus, he writes
These are the Karaites, the Chasidism, and the Zoharites, or followers of Sabbathai Tzevi."
I think it would be interesting to trace the earliest English language reference to Chassidus that there is. I'm on it.