Henry Stacy Marks: Capital and Labor (1874)

In 1871, the Trade Union Act decriminalized trade unions in Great Britain:

The purposes of any trade union shall not, by reason merely that they are in restraint of trade, be deemed to be unlawful so as to render any member of such trade union liable to criminal prosecution for conspiracy or otherwise.

A minimum of seven members was required in order to officially register a union with the government.