Following on from this with the Roman invasion of Britain. The Romans
probably did the same thing before AD 43.
They had their enclaves of Romanised immigrants especially in Sussex. They
never needed to invade Sussex because they were already there!
For the linguists, why are there no Celtic (unless you count Lewes?) place
names in Sussex. Because the Romans got rid of most of them and then the
Saxons got rid of the rest. There are still part Roman place names.
Portslade, Stretham, Southwick, Wyckham, plus.