They appear to quote the guardian. Some US doctors are suggesting the same timeframe.
The Guardian was likely the source for his video.
“The coronavirus epidemic in the UK will last until next spring and could lead to 7.9 million people being hospitalised, a secret Public Health England (PHE) briefing for senior NHS officials reveals.
The document, seen by the Guardian, is the first time health chiefs tackling the virus have admitted that they expect it to circulate for 12 more months and lead to huge extra strain on an already overstretched NHS.
It also suggests that health chiefs are braced for as many as 80% of Britons becoming infected with the coronavirus over that time.”
https://www.theguardian.com/world/2020/mar/15/uk-coronavirus-crisis-to-last-until-spring-2021-and-could-see-79m-hospitalised?