The signs of the times have changed and therefore, even certain decisions differ from one person to another. Even history cannot say it repeats itself when in fact the present situation is entirely different from what happened in the past.
Although there are rules in British Monarchy than all the laws of an ordinary citizen, not to mention the traditions they have to follow, the Royals are but humans, but feelings and free-will. That is why even the British Royal Family are shattered by the occurrence of divorce and separation, and how their laws have to be in conjunction with modern times.
When Edward VIII married Wallis Simpson and abdicated his throne, the couple exiled to France and lived outside the British Monarchy. But before his death in 1972, a royal title was reverted to his behalf and he became Duke of Windsor. From this point, a reconciliation has given to Edward VIII and his wife who is an American socialite and twice a divorcee, causing a stir in the constitution of England.
But ever since being royal and next in line to the throne, Edward VIII was known to have his own decisions and did not care at all to his responsibilities to the throne. Some royal documentaries would tell of Edward VIII to be going for married women and Wallis Simpson specifically was married and was still married to her second husband, despite all these, the former King chooses to be with her.
With this action, fate has pushed Princess Elizabeth's father to be the next King and her to be "Heir Apparent," this sudden turn of events even shocked the young Princess, and thus started her training to be the future Queen of England. Consequently, her future children will be next in line, and Prince Charles being the eldest had to face his destiny too.
However, as previously mentioned, the British Royal Family is not immune to scandal and failed marriages, as Prince Charles and Princess Diana's "fairytale wedding" ended in divorce in 1996. From there the Prince of Wales did not have the sympathy and trust of the people, now that he is set to be King of England.
According to The Succession to the Crown Act of 1707, it states that the monarch has to be a Protestant, and this does not really say if marrying a divorcee or being divorced makes him not suitable to be King or Queen. In addition to that, Queen Elizabeth II has already blessed the second marriage of Prince Charles and the now Duchess of Cornwall.
Under the Royal Marriages Act 1772, the monarch will have to approve of the marriage of a member of the Royal family so that it can guard against those that could "diminish the status of the royal house," in 2013 a change in the law applies only to the six people closest in line to the throne who need the Queen's approval first. This was then applied to Prince William and Prince Harry before they get to ask the hand in marriage to Kate Middleton and Meghan Markle.
Some news source claims that Prince Charles might not want to be King of England now that he is about to retire, fans of the Royal Family are also thinking if he would consider abdicating his throne for his younger brother Prince Andrew, and not for Prince William. But the decision is not really with Prince Charles, as the British Monarchy follows a line of succession and if he wishes his "more-experienced" brother to be King, he should first accept the crown and would have to dictate such wishes in a document called an Instrument of Abdication.