"Historians and genealogists also argue the historical accuracies upon which the claim is built. This claims rests on the idea that Zaida of Seville was the daughter of Abu al-Qasim Muhammed ibn Abbad, who, in turn, was descended from the Prophet (PBUH). However, Zaida’s own origins are debatable as some argue that, rather than being Abu al-Qasim’s daughter, she merely married into his family.

Following the 2012 discovery of Richard III’s remains in a car park in Leicester, various genetic tests have been conducted on the Plantagenet King. Recent research has brought into question his relation to the Queen, suggesting she does not descend from him and therefore, by extension, cannot be a descendant of the Prophet Muhammad (PBUH).

Many have pointed out that “if you trace anyone’s lineage back far enough, you discover – shock horror – we’re all related to each other eventually.” – meaning that there is a significant difference between being directly descended from someone or simply being related to them. The Observer tries to put this in perspective by demonstrating that a child born in 1947, would have had 1,024 ninth-grandparents: compare this to the Queen, who is purported to be the 43rd great-granddaughter of Prophet Muhammad (PBUH). Supposing that this is true, the Queen would have hundreds of thousands of 43rd great-grandparents.

And so with gaps in historical records and no genetic evidence, it is difficult to prove the relation between an 11th-century Muslim princess and a 20th-century British Queen, never mind a 7th-century Prophet."

https://theculturetrip.com/middle-east/articles/is-queen-elizabeth-ii-really-related-to-prophet-muhammad