Did he not say essential work only?
No. And if he did, he'd have to provide a list of what counts as 'essential work'.
This is what BoJo said; I'm a touch typist and typed it as he spoke:
These are the reasons you can leave your house:
Shopping. For basics and kept to a minimum.
One form of exercise per day (run/walk cycle) (though I don't think cycling is exercise. It's cheating. On a cheat machine.
Any medical need (for someone else counts)
And travel to and from work, but only when absolutely necessary, and cannot be done from home.
That's all.