Britain has an official death toll from confirmed COVID-19 cases of over 40,000, rising to over 50,000 cases when deaths from suspected cases are included.
Prime Minister Boris Johnson introduced the lockdown on March 23.
Epidemiologist Neil Ferguson told lawmakers that Britain had taken the right measures against the new coronavirus, but too late.
Ferguson, a professor at Imperial College in London, produced an influential model which influenced Britain's response to the pandemic and showed that unchecked, the virus could result in hundreds of thousands of deaths.
However his later estimate, once lockdown had begun, that 20,000 might die, proved inaccurate because the elderly in care homes were not successfully shielded from the virus, he said.
Ferguson stood down from the Scientific Advisory Group for Emergencies (SAGE), which plays a key role in shaping government policy, in May, after a newspaper reported he had broken lockdown rules by meeting a female friend.