The “official line of succession” is just the people who are eligible according to British law, mostly the Act of Settlement 1701, the Royal Marriages Act 1772, and the Succession to the Crown Act 2013. There is no official list of exactly who that encompasses because there are some sticky situations, particularly farther down the list, that probably would require a court argument to settle. The palace doesn’t want to go to that trouble and expense for something that is going to remain theoretical unless there is a huge catastrophe, but some of the people affected might be willing to start legal proceedings if the palace produced an official list excluding them, so it’s just easier to let sleeping dogs lie.
For example, British law excludes anyone who is or has ever been Catholic from the throne, but how does that apply to people who were baptized Catholic as infants but who were never confirmed or who have taken no actions as adults to adhere to the Roman Church?
Then’s there’s the Albany problem: under the Royal Marriages Act, male-line descendants of a British monarch from George II onward were required to obtain the consent of the current monarch before marrying, but the Duke of Albany, a male-line grandson of Victoria, got in a snit over losing his British titles in the aftermath of the First World War (he also held a German title and served in the German army), and none of his children sought the necessary permission. Did that render their marriages void for purposes of succession? (Note that his daughter married into the Swedish royal family and his grandson is the current king, Carl XVI Gustaf, so officially declaring the king’s mother’s marriage was void could be diplomatically embarrassing.)
Also, the Royal Marriages Act did not apply to “issue of princesses married, or who may marry into foreign families” [i.e., the King of Sweden wouldn’t have to ask the British monarch for permission to marry], but a court battle in the 1950s established that basically every descendant of the Electress Sophia (mother of George I) born before 1949 was a British subject by virtue of the Sophia Naturalization Act 1705. That might mean the King of Sweden was himself a subject of the British Crown and required to obtain his monarch’s consent to marry. Whether that interpretation of the law is correct has never been conclusively settled, and probably never will be.
There are a few other of these sorts of issues lurking down the family tree, so it’s easier for the palace just to list the first few people that they’re sure about, the people who have some perhaps faint chance of succeeding, and not try to figure out the whole list.
Meanwhile, the American genealogist William Addams Reitwiesner published a series of lists giving the full line of people who might have some claim, as of various dates (every twenty years since 1701). He died in 2010; his literary executors updated the list to 1 January 2011 (at same link), listing 5723 people, but I know of no comprehensive listing since then, and obviously subsequent births and deaths render that list increasingly out of date.
