James Boswell, in his Life of Johnson, described how one “Reverend Mr. Palmer, Fellow of Queen’s College, Cambridge,” dined with Boswell and Johnson in 1781. Boswell appended a footnote with some more information about Palmer:
“This unfortunate person, whose full name was Thomas Fysche Palmer, afterwards went to Dundee, in Scotland, where he officiated as minister to a congregation of the sect who called themselves Unitarians, from a notion that they distinctively worship one God, because they deny the mysterious doctrine of the Trinity. They do not advert that the great body of the Christian Church, in maintaining that mystery, maintain also the Unity of the God-head; the ‘Trinity in Unity! — three persons and one God.’ The Church humbly adores the Divinity as exhibited in the holy Scriptures. The Unitarian sect vainly presumes to comprehend and define the Almighty. Mr. Palmer having heated his mind with political speculations, became so much dissatisfied with our excellent Constitution, as to compose, publish, and circulate writings, which were found to be so seditious and dangerous, that upon being found guilty by a Jury, the Court of Justiciary in Scotland sentenced him to transportation for fourteen years. A loud clamour against this sentence was made by some Members of both Houses of Parliament; but both Houses approved of it by a great majority; and he was conveyed to the settlement for convicts in New South Wales. “
In other words, promoting Unitarianism in late eighteenth century Britain was sometimes considered illegal. Further, you could be sent to the penal colony in Australia for that crime. I guess Unitarianism was perceived as a threat to the establishment — not just to the established Church of England, but to the political establishment as well.