https://www.gov.uk/government/public...side-your-home
Point 8 Face coverings - not masks
Probably because if they asked for masks, it would be classed as PPE and the consequences then might open up a can of worms.
So instead they ask us to put a random piece of textile over our mouth and nose, with no defined properties or method at the point in the outbreak where we are trying to get the economy going.