Future-dating it is a sensible move to allow people to get used to the idea, to disseminate the news, and to allow time to obtain the necessary equipment. It will increase compliance compared to an immediate rule change. Bear in mind that only about 5%-10% of people in UK currently are wearing masks in shops (based on my own local observations).
Months ago there were massive shortages of PPE and UK was largely in lockdown with most people only going out to exercise and to shop, not going anywhere else, and not seeing famliy, friends etc.
Now the risk profile of things has changed because people are out and about more.
The masks normal people need to wear aren’t the same masks that medical staff need to wear — the former mainly stop you infecting others, the later keep you safe from others.
