You are assuming "country" and "sovereign state" are synonyms, but that isn't true in general. The word "country" is used to refer to the four first-level subdivisions of the UK - England, Scotland, Northern Ireland and Wales - this is standard and well-established English usage (well, certainly standard British English). The concept of a sovereign state having subdivisions known as "countries" is not unique to the UK, as the following article attests - https://en.wikipedia.org/wiki/Constituent_country