It is now compulsory in England for customers in enclosed public spaces to wear a face covering. These spaces include shops and supermarkets, banks, takeaways, and railway or bus stations and you can be fined up to £100 if you don't wear a covering.
Within these spaces you may occasionally be asked to remove a covering by a member of staff, for example to verify your ID, which is allowed under the goverment's rules.
Some spaces are exempt, for example eat-in restaurants, pubs, hairdressers, gyms, and museums. If you're unsure check before you visit to be sure you don't risk a fine, or make sure you have your mask with you just in case.
There are also personal exemptions for age, health or equality reasons, you can see the government guidance on exemptions here and download exemption graphics for your phone or badges and cards to print.
If you don't have a mask there are lots of guides to making your own online that require varying levels of skill! There are no sew-options if you have an old t-shirt you don't mind cutting up:
Or if you have elastic bands and a bandana:
If you are in other parts of the UK the rules are different:
Or if you are outside the UK check the advice of your local government.