Gamache says that Eleanor and "Henri" de Poitiers were a medieval king and queen of France. Eleanor (of Poitiers and Aquitaine) was first married to King Louis VII of France and subsequently to Henry II of England. There was no "Henri de Poitiers" who was king of France.
When Gamache is talking with Bea Mayer he mentions that when he was in Cambridge, England, he went to the cathedral every week.
Whilst Cambridge has many impressive college chapels, it does not have a cathedral.