Putting aside the name Celt (and Druid) however there does seem to have been a fair degree of homogeny in these islands by the time of Caesar's raid in 55 BC...
"A great many old farmsteads in Britain, today, are on Celtic sites. During his raid on Celtic Britain in 55 B.C., Julius Caesar commented on its high population and numerous farms and cattle. The unifying bond between all the Celtic tribes was their common priesthood, the Druids. Their efforts preserved common culture, religion, history, laws, scholarship, and science. They had paramount authority over every tribal chief and, since their office was sacred, they could move where they wanted, settling disputes and stopping battles by compelling the rival parties to arbitration.
They managed the higher legal system and the courts of appeal, and their colleges in Britain were famous throughout the Continent. Up to twenty years of oral instruction and memorizing was required of a pupil before being admitted into their order. Minstrels and bards were educated by the Druids for similar periods.
Knowledge of the Druids comes directly from classical writers of their time. They were compared to the learned priesthoods of antiquity, the Indian Brahmins, the Pythagoreans, and the Chaldean astronomers of Babylon. Caesar wrote that they,
"know much about the stars and celestial motions, and about the size of the earth and universe, and about the essential nature of things, and about the powers and authority of the immortal gods; and these things they teach to their pupils."
They also taught the traditional doctrine of the soul's immortality. They must have professed detailed knowledge of the workings of reincarnation, for one writer said that they allowed debts incurred in one lifetime to be repaid in the next.
A significant remark of Caesar's was that Druidism originated in Britain, which was its stronghold. Indeed, it has all the appearance of a native religion, being deeply rooted in the primeval native culture. Its myths and heroic legends are related to the ancient holy places of Britain, and they may largely have been adapted from much earlier traditions. In Celtic, as in all previous times, the same holy wells and nature shrines were visited on certain days for their spiritual virtues. The overall pattern of life was scarcely changed. In the course of time, society became more structured and elaborate and the Druid laws more rigid, but the beginning of the Celtic period in Britain was evidently not marked by any major break in tradition."*
* http://www.britannia.com/wonder/michell2.html
