> Btw, Can the brit people name any other candidate to be the queen or the king? From the existing family or a new one?

Theoretically yes through parliament, who approves succession issues in a complex system with precedents that about 3 people understand. William of Orange becoming King is probably the most notable example. Recently there was a curios case where succession laws were change so that females inherit equally, but everyone with the Queen as their head of State hate to change their laws at the same time, had one country not done so, or the wrong gender babies been born at the wrong time along with a few untimely deaths, the countries would have ended up with different Monarchs.

Poll. Who knows how it would work. There was a satirical novel on how it might go https://en.wikipedia.org/wiki/The_Queen_and_I_(novel)