A uprising in the Roman empire does not count as Britain invading Croatia and Slovenia just because an army originating in the Roman province of Britannia landed in the region which, 16 centuries later, is controlled by these two countries.

You can find an epub of the book on Library Genesis. It is organized by current nation states but is (obviously) referring to historical invasions of their present-day territory.

For Croatia, it does mention events in the fourth century, but also the 19th (when they built a base on the island of Vis) and 20th (when they reoccupied it). Slovenia it admits they “have come perilously close to not invading at all” but points out that (in addition to that fourth-century Roman campaign) the UK did occupy an area including the present-day Slovenian town of Sežana from 1945 to 1947.