As per PHE, “Coronaviruses can be spread when people with the virus have close, sustained contact with people who are not infected. This typically means spending more than 15 minutes within two metres of an infected person, such as talking to someone for instance.”
Not only the DfE, but also PHE are wrong? You’re having a mare today 
For what it’s worth I agree that the PHE guidelines may well be too lax. Especially in light of that BMJ journal article that you read. However unlike you, I know that children can and do spread coronavirus in schools, especially teenagers.
So, 2m isn’t enough. 4.6 metres may still be risky. Good to know.