Julius Caesar raided Britain, but he was not emperor. Claudius led the invasion and conquest

The Roman conquest of Britain began effectively in AD 43 under Emperor Claudius, whose general Aulus Plautius served as first governor of Britannia.

Britain was conquered by the Romans. Gaul was conquered at the End of the Gallic Wars (58-50 BC). The conquest of Britain started in 43 AD.

Hadrians wall marks the furthest extent of the Roman conquest of britain(not counting the antonine wall)

The Roman conquest of the Britain was a gradual process, that began effectively in AD 43 under Emperor Claudius.