in 1688 James II was deposed as king of england, and in his place William III of orange was put on the throne. the house of orange is usually thought of as coming from the netherlands, but it is actually an offshoot of the older house of nassua which is a german royal line extending far back in history. the nassau-orange royal house has a long history of insinuating their relatives into the families of various royal lines throughout europe. in fact, it was not uncommon for a princess of this line to marry into another royal family, and shortly afterwards her husband suffered a mysterious death (such as from tainted food), thus creating even more opportunity for the house of nassau. the one family they were unable to penetrate were the romanovs of russia, and we saw what happened to them - everyone hunted down and killed (except perhaps one).
a detailed explanation can be found in jim marrs book "rule by secrecy"
EDIT - this means that all current members of the british royal family are actually of german heritage, which may explain why some prominent members of that family were enthusiastic supporters of hitler during WWII.