The protectionist ideas were more and more criticized in Britain by the 1820s, merchants and manufacturers considering the British industry enough strong to flourished without trade protections against foreigners. Moreover, the liberals had a feeling that maintaining high duties on import might provoke a similar reaction from foreign countries on British goods, a bad thing for British trade. Thus, in 1820, the merchants of London, Manchester, and Glasgow, in fact the largest trading cities in Britain, petitioned the House of Commons to put an end to all duties, an event symbolizing the diffusion of free trade and liberal ideas in Britain, to which J. S. Mill is associated (this petition is actually mentioned in its autobiography). Concretely, it came to the Reciprocity of Duties Act of 1823 that permitted Britain to sign mutual trading agreements with foreign countries on an individual basis.

Source: https://www.parliament.uk/about/living-heritage/transformingsociety/tradeindustry/importexport/overview/freetrade/