In the 1920s and 1930s as many Brits lived in Germany as there were Germans living in England. Amiable comings and goings; trade, social and education; tourist, cultural, sporting exchanges were commonplace. Many Brits had investments in Germany and vice versa.
Much the same could be said of Finland, Romania, Hungary, Italy etc. As a Far Eastern country Japan's ties with the U.S.A were on a similar scale. We all know what happened and goodness knows how many were interned when hostilities broke out.
So hypothetical question: Where would your loyalties lie if either Spain or the Uk were to be governed by a regime hostile to the other?
Imagine; displacement camps holding thousands of Brits in barrack like shelters on the suburnt plains of Spain so the costa coastlines are freed up for sea defences.
It couldn't happen? It has ... throughout history. Right, I am off to make a cup of tea while I contemplate my allegiances, if any.