The British Royal Family are from Germany; not England.
They changed their name from their German Royal name to the House Of Windsor in order to be better accepted by the people of Great Britain.
As royalty has so often done throughout history, their official title is more often than not appended with grandiose hyperbole describing their "omnipotence", etc... .
Hence, in official ceremony, the Queen's long, long list of royal titles is pronounced; one of them being "Queen Of Israel".
They most definitely do not share Davidic lineage.
As to whether or not the Royals demand circumcision of their male line, I have never in my life heard any such thing.
However, it is rumored that Princess Diana attempted to circumcise Prince Charles with a kitchen knife when she heard that he tried to rape one of the Palace's male attendants.
Fortunately for Bonnie Prince Charlie, there was no end to the schmuck, so Diana knew not where to begin cutting.