Translatio Imperii
In the case of Geoffrey of Monmouth’s History of the Kings of Britain this succession is determined as follows:
Troy à Rome à Britain
Here, following Virgil, Geoffrey describes how the Trojan Aeneas founded Rome after the Trojan War. From here, Geoffrey describes how Aeneas’ great-grandson Brutus, after being banished from Rome, came to found Britain. In this way, Geoffrey describes Britain as inheriting the seat of imperial power from Rome. In other words, Geoffrey plays up the importance of Britain in the course of world history by linking it to the great empires of earlier world history.
Perceval has nothing to do with translatio imperii.