Not explicitly. Instead, his/her argument has the form: "Every country claims to be the important country, and nationalists are bad, so it can't be Britain." I

Not explicitly. Instead, his/her argument has the form: "Every country claims to be the important country, and nationalists are bad, so it can't be Britain." I commented because I thought that argument was badly constructed. Of course, anyone can guess whether the argument's author supports the alternative argument (as you claim).