Under English law, any child born to a Royal out of wedlock is ineligible to succeed the throne, which extends to the child's heirs. This can never be reversed, even if the child is legitimated by the Royal parent. Therefore, despite being the "only" surviving member of the Windham line, Ralph could never take the throne since his father was born out of wedlock.
While this movie is fictional, King Ralph and King Cedric would not be titled as Ralph I and Cedric I. They are only numbered as such once there has been a second monarch of the same name. Hence Queen Victoria is not referred to as "Victoria I."
They repeatedly refer to Ralph as the King of England. The Kingdom of England ceased to exist in 1707 during the reign of Queen Anne, therefore the last King of England would have been King William III.
While the film states that the entire British Royal family has been killed and an heir cannot be located, the actual line of succession to the United Kingdom monarchy extends to hundreds of nobles throughout all the nations of Central and Western Europe. An even longer extended list, tracing to links with the royal family as far back as 1800, extends into the thousands. Thus, even if all of the immediate British family were to die, there would be plenty of people left to take their place.