The British royal family has long thought it is the Davidic Dynasty. So do some "British-Israelite" sects throughout the world. This is why they were traditionally circumsized.
Now behave.
I simply cite that it is not common knowledge....It may be what the orthodox community talks about but it is not official policy nor is it expressed by the royals at all.
It is in no way rude to ask someone for real proof as opposed to those rumors that the chattering classes spread. Show me some citation that the British royal family considers itself the Davidic Dynasty....