To be pedantic, it won't be mandatory to wear 'masks' (as in the title of your post) but only to wear 'face coverings'. The official advice has always been (and, as far as I know, still is) that purpose-made face masks should only be worn by those who're in contact with people who're displaying Covid-19 symptoms or by certain other health professionals.
If everyone rushes to buy 'proper' face masks, there's a risk of shortages arising again for those who really need them.
Incidentally, I've been for a chemotherapy session this morning, so I've been in an area full of 'vulnerable' patients (because the very nature of our treatments means that our immune systems are effectively switched off by the chemicals pumped into us). Everyone attending for treatment was invited to wear a face mask (provided free of charge by the hospital) but it was stressed that it was entirely a personal decision and NOT compulsory. I opted not to wear one, as did about 50% of all patients being treated there this morning. (They tend to become rather pointless anyway, with all the free tea and coffee that they keep bringing round; it's a bit difficult to drink it while wearing a mask!)