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I knew that you had a great understanding of the titles (from Downton) but sometimes a question is raised and others have less understanding but are similarly intrigued.
Lord Colin Campbell COULD have prevented her from using her Lady Colin Capblell title upon their divorce, but it would have taken a court order which would have brought disrepute to one of the most high-ranking Scottish families who are clan leaders.
If Lady C had re-married, she would have lost her courtesy title at that time. Lady C chose to have several partners (the most well-known is Larry Lamb - actor from Easters fame). However, when Lady C was in that relationship, he was simply a 'two-bit' actor trying to make good so she couldn't have been going after any money/title.
In the realms of aristocracy, the only rank higher than a Duke is a Royal Duke (for example, HRH The Duke of York, HRH The Duke of Cambridge, HRH The Duke of Cornwall).
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For those who are learning about the hereditary British/Scottish aristocracy ranks of peerage from Downton Abbey, Lord Grantham is the Earl of Grantham and children are bestowed with the title of Lady or Lord as a consequence. If he had had a son himself, that son would have been referred to as The Viscount Grantham. However, Earl is a lower rank than Duke and Marquess which is why there was great excitement when Lady Edith (own title for a life-time) might marry Herbert Pelham, Marquess of Hexham because Edith would 'out-rank' all of the Crawley family who are Earls/Counts/Countesses at formal dinners (Earl and Count is interchangeable because no feminine form for Earl was ever established).
Edith would become the Marchioness of Hexham - although still referred to as 'Lady Edith' by family and close friends. She would be one rank lower than a Duchess and 2 ranks lower than a Royal Duchess. But she would be far, far further up the ladder of aristocracy than Barons, Baronets and Knights - whilst also being ranked above her own father (Earl Grantham) and Master George of Downton who is the heir to Downton Abbey.
Sadly for Marigold, the Succession Act won't be brought in by the time of the DA Christmas Special (1925) 