A further 38,281 cases of Covid-19 have been confirmed in the last 24 hours, along with 140 deaths.
The latest figures mark the the highest number of cases for five weeks and the most new cases since July 22.
It comes as figures published on Thursday by Public Health England estimated the vaccine rollout has directly averted between 102,500 and 109,500 deaths, and more than 82,100 hospital admissions.
Devi Sridhar, professor of global public health at the University of Edinburgh, said vaccine advisers are being "very cautious" on widening the jabs programme but warned that "waiting and watching costs time".
Asked about 12 to 15-year-olds being offered jabs, she told BBC Radio 4's Today programme: "Either you're going to be exposed to Covid without any protection or you can be exposed and have a vaccine.
"And we should be offering teens that vaccine so they have that protection before going back into schools."
Scotland's deputy first minister John Swinney said the return of Scottish schools this term had contributed to a rise in cases in the country.
Prof Sridhar said: "I think the issue is they (the JCVI) are being very cautious.
"They're waiting and watching and I guess the issue with a pandemic is that waiting and watching costs time."
She said not jabbing this age group now would mean "they've lost that window of time".
Prof Sridhar added: "It's an emergency situation and we have Delta, which is so infectious. I mean, it's just flying through schools as we know.
"But not just here, Germany, Denmark, even places like New Zealand and Australia are struggling with Delta compared to the original virus."
A spokesperson for the Department of Health said: "No decisions have been made on vaccinating 12 to 15-year-olds and it is inaccurate to suggest otherwise.
"Ministers have not yet received further advice from the JCVI on this cohort."We continue to plan for a range of scenarios to ensure we are prepared for all eventualities."
Meanwhile, returning secondary school and college students in England are being urged to get tested - and vaccinated where possible - to stop coronavirus spreading and minimise disruption to lessons over the autumn.
Ministers have launched a campaign, backed by an Olympic champion and a TV doctor, to persuade parents, secondary school and college students to take part in voluntary asymptomatic Covid-19 testing.
A school leaders' union has warned that the take-up of twice weekly testing at home was "patchy" during the summer term despite efforts by school staff.
Meanwhile, an executive head of three schools in south Yorkshire has acknowledged that teenagers are likely to be "fed up" with taking Covid tests.
But Andrew Truby, who runs two primary schools in Sheffield and one in Rotherham, urged parents to encourage their children to get tested regularly to enable greater "freedoms" and reduce disruption.
