Why was southern Britain partially Romanized before the Roman invasion of 43 AD?

The south of England was to some extent romanised before the Roman invasion, because, being close to Gaul and the Roman Empire, many tribal kings and nobles engaged in trade with the Romans and had diplomatic relations with Rome. Kings deposed by other tribes sometimes took refuge in Rome. It was the exile of a Briton king by another tribe which gave Claudius the pretext to invade Britannia, claiming that it was aimed at restoring this king. Claudius wanted the glory and prestige of conquest to bolster his weak political position in Rome.

When Rome invaded Britain in 43 AD CE, they found the lowland southeast partially Romanized due to its proximity to Gaul. Rome was not satisfied, however by only occupying this area of Britain. This in itself took four years. This was a thinly populated area. Their rule there was due in part to the help of native princes. Expanding northward toward Scotland and into Wales would require another 30 years. Rome, however, was a patient empire.