Each of the examples you give are either: (a) just two nouns conjoined by the word "and" (a la France and Germany); or (b) an determined reading not to interpret "Britain and Ireland" as another way of saying "British Isles" (contrary to the sources on that page and a la those pedantic arguments that if it doesn't include the Isle of Man then it cannot mean the "British Isles").
WRT your suggestion for a dab page links: UKGBI is about as reasonable as it gets but even when that polity existed "Britain and Ireland" was not a common term for it (a la GB and GBR, not BI or even GBI, for the country code) and it isn't today. In contrast, it is a common name of the topic described at British Isles.
Why not have it as a seperate article on terminology? Because Terminology of the British Isles already exits. Why not redirect to the terminology page? Because the title is an another name for the topic described at British Isles not the one described at Terminology of the British Isles.
Why are you so resistant to it being so when it is supported by secondary sources? Why are you suggesting that it be a dab page, an article of its own, redirected elsewhere ... anything but redirected to British Isles - where even in the lead to that article the phrase is bolded as another term for that topic? RA