Do you know whether discrimination law applies to individual employees or to the business / owner? Because a shop owner who insists on enforcing mask wearing could put their employees in a very difficult position indeed.
They could.
It applies to both employers and employees (and even agents where money is exchanged). If it can be linked to the course of employment the employer may be found responsible (most often). The employee is responsible for all acts of discrimination although if they were told to do it by the employer and that it was correct and reasonably believed it was the right thing to do they would not be (unfortunately this does crop up). In practice there are all sorts of scenarios like in big companies where the employer turns on the employee, even if custom and practice or orders were given to do the wrong thing to the employee. Or it's just very unpleasant. Or rumours go around, or publicity.
Although stopping coronavirus wouldn't be disputed as a legitimate aim, the bit that's important is whether it's proportionate. Retail culture and other environments are not known for their sage and well planned health and safety and welfare of employees unfortunately. Outside the business, the theatre of public opinion, the media and the courts tend to take a much dimmer view. Especially as it's probably most likely to happen in a food shop, it's not likely to be regarded as proportionate.
Having said that, I do think some service providers such as councils and mental health services have not set a good example on discrimination and have just stuck two fingers up at some quite at vulnerable groups and these things also have an effect how the rest of business/society behaves, sadly.
