Here is a timeline for Britain before the Romans arrived, from the earliest human presence and the arrival of Homo Sapiens.

c700-800,000^BC
The earliest humanoid presence in Britain at Happisburgh.
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c485,000^BC
Boxgrove Man (Homo heidelbergensis) – the oldest humanoid fossil found in Britain so far.
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c350,000^BC
Swanscombe woman.
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c25,000^BC
Arrival of Homo Sapiens – our early ancestors.
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c12-10,000^BC
Permanent human settlement begins.
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c8,000-6,000^BC
Last Ice Age ends in Europe.
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c6,000^BC
Last land bridge separating Britain from the rest of Europe was swept away.
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c4,000-2,500^BC
Neolithic period – the first farmers, possibly arrivals from the east.
Early enclosures and burial chambers.
Later Neolithic period sees the construction of henges and stone circles.
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c3,000^BC
Construction of Stonehenge begun.
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c2,500-750^BC
Bronze Age – round barrows (burial chambers), villages and the first hill forts are built.
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c1,200^BC
Construction of roundhouses.
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c750^BC
The Iron Age.
The arrival of the Celts, probably in waves.  Hill forts are a typical feature of settlement.
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c500^BC
Construction of brochs in Scotland.
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c300^BC
Somerset lake villages.
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c100^BC
Coins are in use.
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55^BC
Immediately before the Roman invasion, Britain is peopled by a number of different tribes.  Their names come to us from Roman writers and include the Durotriges in modern Dorset/Wiltshire, the Atrebates in parts of Sussex, Hampshire and Berkshire, the Iceni in Norfolk and Suffolk, the Brigantes in Yorkshire, Durham and Lancashire, the Votadini in south east Scotland, the Caledones in the Highlands and the Picts in north east Scotland.
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55 and 54^BC
Julius Caesar invades southern Britain.  He exacts tributes from defeated tribes but does not stay.
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43^AD
Full-scale Roman conquest of Britain begins.