It's a pretty interesting scenario, TBH, and it does remind me of Lord Grattan's TCoH in a way. However, though, I'm not sure if a 1649 PoD would be sufficient.
Supposing that the royalty did not move to Virginia and remained in a European court but the colony still declared for them how would it develop ?
I don't think Thoresby's idea would work, though, simply because the royal families weren't this one giant family tree as some people tend to think. Not to menetion that the Stuarts were Protestants and most of Europe was still Catholic at that point, even much of Germany where the sect was originally born......so, undoubtedly there would be problems.
Unless the opposition forced the royal family to go to the backwater and kept them under house arrest.
That isn't quite so farfetched when you think about it. If the Commonwealth manages to not only gain control of Britain permanently, there might be a lot more people willing to emigrate outwards than there were IOTL, and with that in mind, some very interesting long-term developments could occur....now, whether this kingdom ends up being more like the Draka or TL-191's C.S.A., or more like Sobel's C.N.A. or TCoH's *America is really anyone's guess, IMHO, but regardless, I'd probably be interested if anyone were to attempt a TL based on that premise.