Hillier, who reportedly had underlying health issues, died in hospital on Sunday, having been moved there from the category C male sex offenders' prison.
"As of 5pm [1700 GMT] on 26 March 2020, 578 patients in the United Kingdom who tested positive for coronavirus (COVID-19) have died", said the official government website, up from 463 on Wednesday.
But in a series of Facebook posts, Ms Middleton's family urged the public to heed safety advice and take the virus "seriously". My loved ones are going through the most unimagineable pain.
Diane Middleton her devastated mother wrote on Facebook the following "To all the people out there that thinks it's just a virus please think again".
She died on 21 March and was from High Wycombe, Buckinghamshire.
"Speaking from a personal experience this so called virus has taken the life of my 21-year-old daughter".
Prime Minister Boris Johnson ordered Britons on Monday to stay at home to halt the spread of the virus, imposing curbs on everyday life without precedent in peacetime. Please, please adhere to government guidelines. Do your bit. Protect yourselves and protect others! This virus could put you in hospital for weeks, or even kill you.
She added, "About time people took this seriously, before too many people end up in this devastating position!"
A total of 422 patients who tested positive for coronavirus had died in the United Kingdom as of 1pm on Tuesday, the NHS said.
Last week, Prime Minister Boris Johnson ordered pubs, restaurants and almost all shops to close, banned social gatherings and told people to stay at home unless they needed to buy food, go out to essential work or to exercise once a day.
The UN's health body warned this week about how young people are also at risk from the virus.