Day 2 Story 3 of the Decameron is a story which deeply involves the English royal family. I don't know how much contact there was between England and Italy in the 14th century, or how much care an Italian writer of that time would have been expected to take in providing accurate historical information about foreign countries, but how much of the described English events have any basis in real history?
There is a war between the English king and his son, which lasts for several years:
While the three brothers thus continued to spend freely, and, when short of money, to borrow it, never doubting of help from England, it so happened that, to the surprise of everybody, there broke out in England a war between the King and his son, by which the whole island was divided into two camps; whereby Alessandro lost all his mortgages of the baronial castles and every other source of income whatsoever. [...] Alessandro, meanwhile, seeing that the peace, which he had for several years awaited in England, did not come, and deeming that he would hazard his life to no purpose by tarrying longer in the country, made up his mind to return to Italy.
A daughter of the English king travels to the pope in Rome, disguised as an abbot, in order to escape marriage to the king of Scotland:
“Holy Father, as you must know better than any other, whoso intends to lead a true and honourable life ought, as far as may be, to shun all occasion of error; for which cause I, having a mind to live honourably, did, the better to accomplish my purpose, assume the habit in which you see me, and depart by stealth from the court of my father, the King of England, who was minded to marry me, young as you see me to be, to the aged King of Scotland; and, carrying with me not a little of his treasure, set my face hitherward that your Holiness might bestow me in marriage. Nor was it the age of the King of Scotland that moved me to flee so much as fear lest the frailty of my youth should, were I married to him, betray me to commit some breach of divine law, and sully the honour of my father's royal blood.
She marries an Italian (one of the major characters of this story) who becomes the earl of Cornwall and later, perhaps, the king of Scotland:
The two knights went before them to England, and by their influence induced the King to restore the lady to his favour, and receive her and his son-in-law with every circumstance of joy and honour. Alessandro he soon afterwards knighted with unwonted ceremony, and bestowed on him the earldom of Cornwall. And such was the Earl's consequence and influence at court that he restored peace between father and son, thereby conferring a great boon on the island and gaining the love and esteem of all the people. Agolante, whom he knighted, recovered all the outstanding debts in full, and returned to Florence immensely rich. The Earl passed the rest of his days with his lady in great renown. Indeed there are those who say, that with the help of his father-in-law he effected by his policy and valour the conquest of Scotland, and was crowned king of that country.
I'm fairly sure that at least the last of these points doesn't have any basis in reality, but maybe the first one does.