The UK, Canada, Australia, etc... all were (and still are) Constitutional Monarchies. That is their consitution. What you are getting at, I believe, is the executive designation, which in those countries was (and is) "Parliamentarianism", and this is the case in-game. This basically means that parliament is superior, and hence the monarch can't (or rather, doesn't) dismiss the parliament.
As for the lack of migrants to the colonies, this is because less POPs are made to be out of work like they are in vanilla. This is because RGO level is higher, meaning more POPs are in employment, and thus less likely to migrate. In addition, releasing the colonies reduces the places where colonial migration can occur (since the UK no longer owns it).