Repeal the Corn Laws is a decision unique to 
United Kingdom.
The only requirement is that United Kingdom has an average Militancy of at least 2.
It gives 1 consciousness and removes 1 militancy. Additionally it gives the country modifier 'corn laws repealed' for 10 years, which gives -0.02 militancy and 0.01 consciousness pr. month. Additionally the modifier limits the maximum possible tariffs by 5%.
It is worth it to take, if UK's militancy is high and consciousness is relatively low. The maximum tariffs is less of a problem, unless the ruling party has 'free trade' as it trade party issue. If it is not so, the maximum tariffs is limited to 95%, which should more than suffice.
Ingame text
The 'corn laws' were a series of vastly unpopular tariffs, often described as a rare example of British mercantilism, designed to 'protect' British farmers and corn prices from cheap foreign competition, which, naturally, had the effect of effectively increasing corn prices for Britain's poor. After the failed harvest of 1845, and the following Potato Famine in Ireland, the Corn Laws were finally repealed in 1846. The failure of these tariffs heralded a long period of British commitment to the ideals of Laissez-faire capitalism, that was to last for the most of the 19th century