Yeah but those families still exist today... and have been politically manipulated... excluding Catholics for example- namely the Stewarts. That is not including the constant challenges throughout the Wars of the Roses and the Tudors later on. There's that guy in Australia who has a better claim than the Queen... some would even say the Beaufort's do as Plantagenets.
No Welsh Prince or King has ever ruled the entirety of the British people... what is currently Wales sure... but Strathclyde still existed between 1050-1100 which seemed to still have its own King in 1054 at least - and at that time Cumbrians were very much the same people.
And again... unlike the English our King's... as in King's of the Briton's were elected by their peers.