Great Britain currently houses a trio of three countries under the union (Wales, Scotland, England). The isle first had the Celts, then the Romans came but never conquered Caledonia (aka Scotland), and William the Conqueror only conquered England (Norman stuff in Wales, etc.). I am wondering if the isle could ever be considered fully conquered by a foreign power.
I think calling it "never conquered" would be misleading at best. You'd really need to add "never fully conquered by the same people at once assuming you don't count the Glorious Revolution". Different parts of the island have been conquered at different times, frequently by outsiders. Let's make a brief list:
Romans - England, Wales, and the southern half of Scotland up to about the level of the Firth of Forth, where the Antonine Wall was.
Angles, Saxons, and Jutes - arguably a conquest, how violent the settlement was is to my knowledge unclear, covers eastern half of England, some areas of Cornwall, some areas of Scotland.
Vikings - Danelaw stretching across England, but including Welsh and Scottish rulers as vassals. Arguably covering entire extent of the island, depending on what you want to call conquest.
William 'the Conqueror' - needs no explanation, conquered England and installed a new nobility
Edward I 'Longshanks' - conquered Wales, and extracted a recognition from the Scottish magnates that he was their feudal overlord (though the Treaty of Falaise had done something similar before, with William 'The Lion' swearing fealty to Henry II). If you technically rule the whole of Britain, does that infer a kind of conquest? Not Norman, Plantagenet. A foreigner? He spoke in French and owned extensive French lands, it depends how important you think his connection to people before the Norman conquest was.
Glorious Revolution of 1688 - William III, the Dutch Stadtholder, rises to power after overthrowing James II. Gains crowns of England, Ireland, and Scotland after swift acceptance by Parliament (and the fleeing of James II), so he doesn't have to shed much blood to be king until the later Jacobite rebellions (which are put down with force). Under a medieval monarch, this would undeniably be a conquest. Is it in the 17th century if Parliament has accepted it? I think so.
Is Great Britain 'The Isle Never Conquered'? No. It's been conquered in a lot of its areas by a lot of different people, and in the 17th century all at once by the same guy.