"Great Britiain, of course, (U.K. of), was always solidly Protestant."

Perhaps I misunderstand, but before protestantism Great Britain was catholic. And even after Henry VIII and the victory of Cromwell and William, large part of the population remained catholic, although with a lesser status in society (not to mention Ireland).

The reference was to the United Kingdom of Great Britain which, as a state that came into being from 1707, was solidly Protestant, the component kingdoms having ejected ther shared Catholic monarch in 1688. Those component kingdoms, Scotland and England, had both been effectively Protestant since the mid-C16th. There did however remain enclaves of loyalty to Rome in both kingdoms, in Lancashire and the Highlands of Scotland for instance, but these comprised a minority.

All western Europe had indeed been Catholic before the reformations of the C16th, although there were non-comformist sects in evidence throughout the later Middle Ages: Cathars; Lollards, Hussites, etc.