Carlile was sentenced in 1821 to three years' imprisonment for blasphemy, and Watson went up to London in September 1822 to serve as a volunteer assistant in his Water Lane bookshop.

In February 1823 Watson was charged with selling a copy of Elihu Palmer's Principles of Nature to a police agent, spoke in his own defence, and was sent to Coldbath Fields Prison for a year.

In prison he read David Hume, Edward Gibbon, and Johann Lorenz von Mosheim's Ecclesiastical History, and developed his anti-Christian and republican opinions.

Julian Hibbert, an admirer, died in January 1834 and left him a legacy, with which Watson enlarged his printing plant.

He started by printing the life and works of Tom Paine, and these volumes were followed by Mirabaud's System of Nature and Volney's Ruins.

Later he printed Lord Byron's Cain and The Vision of Judgment, Percy Bysshe Shelley's Queen Mab and The Masque of Anarchy, and Clark on the Miracles of Christ.

In 1832 Watson was arrested, but escaped imprisonment, for organising a procession and a feast on the day the government had ordained a "general fast" on account of the cholera epidemic.

In February 1833 he was summoned at Bow Street for selling Henry Hetherington's Poor Man's Guardian, and was sentenced to six months' imprisonment at Clerkenwell.

A grey granite obelisk erected by friends commemorated his "brave efforts to secure the rights of free speech".