Impact of the Report
The New York Times credits the reports from Imperial College  for having a particularly influential impact on policy:
The report, which warned that an uncontrolled spread of the disease could cause as many as 510,000 deaths in Britain, triggered a sudden shift in the government’s comparatively relaxed response to the virus.
American officials said the report, which projected up to 2.2 million deaths in the United States from such a spread, also influenced the White House to strengthen its measures to isolate members of the public.
Imperial College has advised the government on its response to previous epidemics, including SARS, avian flu and swine flu. With ties to the World Health Organization and a team of 50 scientists, led by a prominent epidemiologist, Neil Ferguson, Imperial is treated as a sort of gold standard, its mathematical models feeding directly into government policies.
Imperial College, experts noted, was part of the advisory group for the government’s now-abandoned strategy, which played down radical social distancing and accepted that the infection would spread through the population. The theory is that this would build up so-called “herd immunity,” so that the public would be more resistant in the face of a second wave of infections next winter.
But such a strategy, the report noted, would lead to a flood of critically ill patients in a country without enough beds. Instead, it said, Britain needs to pursue “suppression,” which involves far stricter lockdowns, like the closing of schools and the quarantine of infected people and their families. That would drive down the number of cases and spread out the flow of patients over a longer period, allowing hospitals to cope.
Dr. Ferguson has been candid that the report reached new conclusions because of the latest data from Italy, which has seen a spiraling rate of infections, swamping hospitals and forcing doctors to make agonizing decisions about who to treat. 
Thus the report played a significant part in shifting the Government’s policy from imposing no controls in the hope that herd immunity from social distancing and hand-washing would protect the public, to the highly socially and economically disruptive ‘lockdown’ policy that remains in place today [17 April April 2020].
On 24th March 2020 the following article appeared on the parliament.uk website:
Matt Hancock [the UK Health Minister] has made a statement on the lockdown measures introduced by the UK Government yesterday, which aims to slow the spread of coronavirus.
On 23 March 2020, the Government announced measures to stem the coronavirus pandemic. These included a ‘lockdown’: citizens should now stay at home apart from essential travel or risk fines and all non-essential shops were to close.
Today, [4th March 2020] the Health Secretary updated MPs on these measures. [He] told MPs that people should only be leaving their houses for four reasons:
*to buy food *to exercise once a day alone or with members of the same household *because of a medical need or to provide care for a vulnerable person *to travel to or from work that cannot be done from home. For work that cannot be done at home, Mr Hancock recommended a 2 metre gap between employees.
The Minister said that any non-essential shops must close and that gatherings of more than two people were not allowed. He stressed that “these measures are not advice. They are rules and will be enforced, including by the police”, and noted that fines will be issued for those that do not comply.
He also updated MPs on “shielding”. This is comprised of the Government writing to up to 1.5 million of the most vulnerable people in the UK who need to “shield themselves” to advise them on the supplies and support they need. [He] concluded: “Home is now the frontline and in this national effort, working together, we can defeat this disease. Everyone has a part to play.” 
Imperial College London is highly influential and the reports of its COVID-19 Response Team had a direct influence in changing UK Government policy from herd immunity to lockdown.
What do you think
This is a story of the scientific analysis evolving with the pandemic. Should we expect our scientists to get it right first time? Should we give them the luxury of changing their minds as more information comes in? Or is it inevitable that a new virus like COVID-19 will create great uncertainty and that we should be grateful to our scientists for applying the best mathematical scientific techniques to give the best understanding possible at the time? Was the British Government over-optimistic in initially pursuing a herd immunity strategy? Or are the lockdown policies doing more damage than letting the epidemic take its course? Is it a good thing that scientists and policy makers should be willing to change their minds as more evidence becomes available?
 The Imperial College COVID-19 Response Team, Report 12 - The global impact of COVID-19 and strategies for mitigation and suppression,, Imperial College London, 26th March 2020. https://spiral.imperial.ac.uk:8443/bitstream/10044/1/77735/10/2020-03-26-COVID19-Report-12.pdf
 Mark Landler and Stephen Castle, ‘Behind the Virus Report That Jarred the U.S. and the U.K. to Action’, New York Times, Published March 17, 2020, Updated April 2, 2020 https://www.nytimes.com/2020/03/17/world/europe/coronavirus-imperial-college-johnson.html
 parliament.uk, ‘Health Secretary gives update on new ‘lockdown’ measures announced to tackle coronavirus’, https://www.parliament.uk/business/news/2020/march/update-on-coronavirus-24-march-2020/