Seldom a week passes without a comedian raising a laugh by referring to the Royal Family as German (usually on the News Quiz or Have I Got News For You). It is notable, I think, that it is most popular with the generation of alternative comedians who at one stage were a bit edgy, daring and anti-Establishment; it is the last vestiage of the far-off days when hatred of Thatch and Tebbit was an essential qualificatiaon for success. The joke's fulklest form is found in Elton and Curtis' Blackadder Goes Forth, when Captain Darling, accused of being a spy, insists that he is as British as Queen Victoria ("You mean your father's half German, you're half German and you married a German" says Blackadder).
The odd thing is that these comedians would in normal circumstances distance themselves emphatically from lampooning a lifelong UK resident on the grounds that some of their ancestors were born abroad. Particularly Ben Elton, whose father was German, Curtis, who is from New Zealand, and David Baddiel (half German, quarter Russian).
My calculations are that Victoria was 1/4 British, since her mother was German and her father half-German; Edward VII was 1/8 British, George V 1/8 British, George VI 1/16 British; Elizabeth II is the first Windsor Royal to be more than half British (17/32 British).