So how did Simon (VI) de Montfort come to settle in England in the first place? The answer is a cliché: location, location, location.
His great-grandfather Simon (III) was the count of Evreux, a Norman lordship that straddled the border with Île-de-France. That made him a vassal of the king of England, Henry II. He was also the lord of Montfort, which made him a vassal of the king of France, Louis VII. When Henry and Louis got into it, Simon sided with Henry. He was rewarded with plum marriages for his three children: the oldest Amaury (V) married Mabel, the daughter of the earl of Gloucester, the next-born Simon (IV) to Amicia, daughter of the earl of Leicester, and the youngest Bertrade to the earl of Chester.
Amicia went to live in France. In 1204, her brother Robert, now the earl of Leicester, died without issue, and Amicia and her sister Margaret stood to inherit his earldom. It was her son Simon (V), just returned from the Fourth Crusade, who went to England in 1206 to claim her half of it plus the title. King John granted it to him and Simon (V) de Montfort became the fifth earl of Leicester. John then turned around and confiscated it from him because of unpaid debts, presumably some outrageously high fine (inheritance tax) that Simon refused to pay for taking possession of the earldom.
Within three years Simon (V) was in command of the Albigensian Crusade, and in 1215 Pope Innocent III urged his new vassal King John to return Leicester to Simon. John only agreed to commit the earldom to the custody of Simon’s cousin Ranulf, the earl of Chester. Simon (V) was killed outside Toulouse in 1218. His son Amaury (VII) petitioned Henry III for the earldom, but Amaury was a French peer and constable of France. Too much conflict of interest. So in 1230 Amaury’s younger brother Simon (VI) went to England and made the petition for himself. Henry’s reply was ‘talk to Ranulf’.
Ranulf had no children and he was already the earl of Chester and Lincoln, so what the hell. Of course, nothing was for free in this world and Simon agreed to give him £200 for his quitclaim. This was on top of the £500 he agreed to give Amaury. In 1231, Henry was in Wales when Ranulf arrived with Simon and said everything was square between them. Henry accepted Simon’s homage and his family’s half of Leicester was his (the other half was still held by his great-aunt Margaret). Henry did not, however, confer the title.