I'm not sure what that has to do with it. No doubt if they were all British you would be leaping on him for being an isolationist Little Englander Vrexiteer.
As I understand it, racism involves hostility towards a person or persons based on their ethnicity.
On that basis, the argument that this shop is racist simply doesn't fly.
I love the comment on Facebook - why did you call your shop Really British apart from the fact that you sell British goods? Er, what other reason would there be?
So as others have advised, this is people with too much time and a lack of understanding of what racism actually is, making a noise for no good reason.