Pleasingly, almost everyone there was wearing one when I shopped yesterday. There were three exceptions:
1) A woman on her own, who looked the kind of person who wouldn't have ever worn a mask but could claim some kind of hidden disability if challenged.
2) A woman in charge of around four children (none of whom were wearing masks, but children never did have to wear masks); she was more the kind of shopper encountered in Asda than Sainsbury's, so perhaps the culture is different there.
3) A woman who was avoiding eye contact with everyone; my guess is that she forgot to bring a mask with her and was feeling embarrassed.
Sainsbury's staff were all wearing masks, although not always well (not covering their nose) and not always effectively (those transparent face visors wouldn't even keep insects out, let alone viruses).
I shall keep wearing a mask in shops and on public transport for as long as it's socially acceptable to do so. Regardless of COVID-19, if they merely stop me from catching regular colds then they're worth it.