Here’s my take on the wearing of some form of face-covering becoming mandatory, not solely in confined spaces like the tube or trains...
My local Sainsburys in Welwyn Garden City is in the town centre, unlike the Morrison’s store, which is out of town. Both have large car parks; both have put in place controls on how many people can be in the store by putting queueing systems in place, with which people are complying.
The Morrisons store is in a place where there are no other retailers, so people can queue around the perimeter of the premises. The Sainsburys store, however, has people form a queue past several neighbouring retailers, sometimes snaking back around a corner, past the local Superdrug. The other retailers premises are all closed, so the 2-metre rule is easy to maintain.
Now imagine if all those other retailers are allowed to re-open. How do you form individual queues for each shop whilst simultaneously maintaining 2 metres distance between individual lines of people. IT’S NOT PHYSICALLY POSSIBLE! There are other businesses directly opposite the shops past which the current queue for Sainsburys meanders, adding to the problem.
There must be many many urban shopping areas where the local food outlet is able to deploy similar queues solely because the other shops around their own are currently closed. Any wholesale relaxation of lockdown for ‘non-essential’ retailers will render social distancing physically impossible because of space constraints. Which - I believe - makes the wearing of face-coverings essential... not to protect the wearer; but to limit the distance micro-droplets they might emit can travel, thus giving a small degree of protection to those nearby