I fell into the wikipedia loophole, and from reading about Belize, i ended up deep, deep in the Royal Family, most people had "X in line for British throne", how is it decided who gets what place, and who decides it?
TL;DR: There's no "decision", or rather, it was made in 1701 via the Act of Settlement of 1701 as amended. The order of succession is the sequence of heirs of the body lawfully begotten of Sophia, Electress of Hanover (1630-1714).
It was originally
the Crown and Regall Government ... shall be remain and continue to the said most Excellent Princess Sophia and the Heirs of Her Body being Protestants
(then being male-preference primogeniture) but
all and every Person and Persons who shall or may take or inherit the said Crown by vertue of the Limitation of this present Act and is are or shall be reconciled to or shall hold Communion with the See or Church of Rome or shall profess the Popish Religion or shall marry a Papist
are considered legally dead. The monarch had to be in communion with the Church of England, but it does not appear to be a bar in the same way as following the Scarlet Woman.
There have been modifications. In 1936, Edward VIII wanted to leave the throne, but that was illegal: he was the "Heir of Her Body being Protestant" so he was king, period. An act of Parliament was necessary to declare him dead for purposes of the succession ("demise of the crown") and that the Royal Marriages Act of 1772 did not apply to him.
More fundamentally, the Perth Agreement lead to the UK's Succession to the Crown Act 2013 and similar laws in her Majesty's other dominions.
And a few incidental bits dealing with things like marriages before this act, rewriting the Treason Act of 1351 to refer to the heir rather than just the eldest son, altering the Regency Act of 1937 to disallow anyone with a disapproved marriage, et cetera.
In order to be eligible to inherit, you must be descended from Sophia of Hanover and neither Catholic not married to a Catholic.
The current system is primogeniture. The oldest child inherits regardless of sex. Unlike some such systems, descendents of the monarch's oldest child take priority over the oldest living child if he has a deceased older sibling. For that reason, the Duke of York, formerly second in line and now sixth, would not inherit instead of the Prince of Wales if he died unless the Prince of Wales's older son, grandchildren, and younger son died or became ineligible as well. Further down the line, it gets determined by applying this to descendants of deceased ancestors of the monarch.