Without a catalyst, it is difficult to encourage individuals to take collective action in the broader interest of society. A lockdown provides the motivation needed for the majority of citizens to act in the interest of public health. 
Societies often act in self-interest after weighing the social costs. With an abstract threat like a viral pandemic, it can be challenging to motivate the public to bear the costs of preventing the spread of the disease. With a lockdown in place, it limits the perceived social cost of individual action against the spread of the virus. The public no longer has to weigh the costs of social distancing, avoiding large gatherings, and travel. Without a lockdown in place, it still provides the illusion of choice in how individuals respond to the pandemic, despite many social behaviors risking further transmission of the virus. In many ways, this justification for the lockdown solves the dilemma of social coordination amid a public health crisis. Politicians, scientists, and the press are limited in their ability to motivate the public to take action rhetorically. Thus, the lockdown creates a more natural path to social coordination in the public health response.  While it may subvert the individual interests of many, it is necessary to ensure that public health is maintained.
A lockdown may provide the utility of gaining public acceptance for a public health measure, but it is undemocratic. The people were circumnavigated in the decision to lock down the UK. Public officials never asked the people if it was what they wanted. While MPs are elected by the people to represent them, they ultimately must answer to the public. The paternalistic decision to restrict and police the movements of the public subverts their personal liberty. It displays an elitist distrust of the people and a misplaced faith in the bureaucrats that do the bidding of the most powerful.
[P1] Collectively determined outcomes need collectively determined mechanisms to be legitimate. [P2] The lockdown is a subtle means of coordinating the public to prevent the spread of the virus.
Rejecting the premises
[Rejecting P1] While a lockdown may create collective action, it subverts collective control. [Rejecting P2] The lockdown is a symptom of a technocratic elitism that does not trust the will of the people.