If everyone wears one it will surely help slow the transmission by minimising the droplets spread by each wearer. If there is a chance it will help, surely it's worth trying? I wouldn't want to be crammed on a tube without one.
I'm in France, where the lockdown is much stricter and the sale of masks is now illegal as they've all been requisitioned for health workers. However the government wants everybody to wear a mask when they're in busy areas when they relax lockdown, hopefully on 11 May. Recognising that it's impossible to buy a mask and that not everybody can sew or has materials, there is a big drive to co-ordinate makers with companies who can supply fabric, so that masks will be made by those who know how to for the benefit of the whole country. They will be distributed at supermarkets in the first instance as it's considered a good way of reaching maximum people with minimum effort. (In fact plenty of people are already wearing masks, including medical ones
)
Incidentally I reckon I could sew a mask - I have a machine gathering dust that I made a top with about 6 years ago... but even I can follow a pattern and stitch straight lines. I could do it by hand too, don't most people know how to sew? Will probably cut up old shirts of DH's for this as they are the close woven cotton fabric that is being widely suggested.
My local hospital (actually in the next-door country) is reopening slowly for routine appointments and they are also asking patients to wear their own masks as they don't have enough to give out all their patients. Home-made.masks will be needed for this too.
So two European countries where I know what the rules will be, plus all the others I've read about on the news, all telling people to wear masks. And the U.K. going against the grain again, big surprise 