It won’t be. Shops can already ask people to leave for a wide variety of things, with only a small number of protected things they can’t exclude people for.
That's why I said "in all likelihood" and used the word "discriminate".
It'd have to be challenged in court, but it's likely that under Article 9 of the HRA that not wearing a mask could be seen as 'freedom of thought or belief' and would be as illegal as denying entry to someone wearing a turban. The reason is because the mandation is in effect because of public health and safety; once that has been removed then tacitly it is stated that not wearing a mask is no longer an automatic threat to those things.
Where it could be challenged is if a business could say that the rule applied equally to everyone; however, it doesn't, because we already have medical exemptions to mask wearing, so even now it's illegal to blanket ban people from a shop for not wearing a mask.
One thing is absolutely for sure - Waterstones or any other shops cannot legally mandate people not to wear masks in their store from 19 July.