James VI and I was not “formerly” King of Scots. He was King of Scots, then became King of England and King of Ireland in addition to holding the Scottish throne, all at the same time. He did style himself as “King of Great Britain”, but legally the Kingdoms were separate nations with separate parliaments until 1707.
James VI and I was not Roman Catholic, he was raised as a Protestant. His son Charles I was Protestant, but married to a Roman Catholic. His grand children were mixed, some Catholic, some Protestant.
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James II was at minimum a 'closet' Catholic, and so declared upon his deathbed. I had not looked up Charles II, but had presumed due to his opposition to Cromwell and the Commonwealth that he too was Catholic.
James I might have been raised Protestant, but if he was in fact a Protestant as an adult, he was certainly a very curious one. Even the most casual reading of the history of the period shows a number of actions James took that were favoured by the Catholic population and heartily disapproved by the Anglican side. (I could make a cheap joke here about his banning 'the gawff' on Sundays...but there were numerous others.)