If two countries share an island, are they both considered island countries?

England is not split. The island is called Great Britain and together with Northern Ireland makes up the sovereign state called the United Kingdom (UK). Great Britain consists of three nations: England, Scotland and Wales.

There are other islands in the British Isles that are also classed as England, many hundreds of them with most small and unihabited. However, there is the Isle of Wight and the Isles of Scily, which come to mind, which are among the larger and inhabited.

England, together with Scotland and Wales, making up Great Britain, and Northern Ireland, on the island of Ireland, making the UK does consider itself an island nation. I believe nowhere in this country (I live in the UK) is further than 75 miles from the nearest coast. I can be at the coast (if I were allowed but currently am not) within about forty-five minutes drive.