Short answers, pulled from the story.
Pytheas, a Greek geographer sailing between 330 and 320 BC, was the first to record the existence of these people. His own writings have not survived but later writers referred to them as Pretanoí or Bretanoí.
Romans fortified the northern border with Hadrian's Wall in 122 AD. This structure spanned what is now Northern England before they pushed north again to begin construction of the Antonine Wall in 142 AD.
Common Brittonic developed into Welsh in Wales, Cumbric language in Hen Ogledd, Cornish in Cornwall, and Breton in Armorica. Pictish also descended from Common Brittonic rather than being a separate Celtic language.
Major tribes included the Atrebatēs with capital Calleva Atrebatum, Brigantēs/Brigantī holding Isurium Brigantum, Catuvellaunī ruling from Verulamium, and Corieltauvī governing Ratae Corieltauvorum. The Dumnonīī controlled Isca Dumnoniorum while Durotrigēs used Durnovaria and Maiden Castle as strongholds.
A major archaeogenetics study published in 2021 uncovered migration into southern Britain over 500 years from 1,300 BC to 800 BC. These migrants were genetically most similar to ancient individuals from France and their marker swiftly spread through southern Britain between 1000 and 875 BC.