The first immigrants to the British Isles was Iberians. Migrated from Spain/Portugal, they arrived c.2500 BC with the late Stone-Age weapons.
In 43 A.D, Roman came to invade Celtic Society.
THE HISTORY OF BRITAIN
British Social Institution
Then, on c. 800-600 BC, Celts came from the Southern Europe. Their two main groups were Brythons and Gaels. The Celtic society were individual clans loyal to Cheiftains. They also have Druids as their religious leader.
They came twice to Britain. They brought soldiers with their weapons for the first time and brought their society ; farmers, etc ; for the second time. They also brought the improvements and Christianity to Britain.