In the 1970s Britain came perilously close to a coup d’état. Preparations were made for the eventuality that left-wing groups, mostly associated with the union (but not British Labour party) movement, would cripple the country.
Some of the activity around the counter-revolution was paranoid in nature. A whole conspiracy theory had been woven around Harold Wilson, who had been in and out of power from 1964-1976. In other ways, there was a real danger that economic strikes and popular protests could cripple the country.
Figures such as the prominent military figure General Walter Walker coalesced around the person of Lord Mountbatten. Many private militias were formed. The purpose was to seize control of key infrastructure in the event of chaos. The situation had reached such a panic that by January 1974 the army was deployed at Heathrow airport in a show of force.
Were such moves legal? While they were radical and seemed to undermine democracy, it is a duty under English law for private citizens to maintain law and order. If such moves were intended to preserve the status quo then they were not illegal.
See more by watching this 2006 BBC documentary.