There seems to be more continuity in Central Britain/Northern England than in the South; Sutton Bank hill fort dates from around 400BC, and Almondbury was apparently burned down shortly before that, as if [warning: conjecture follows] there had been an unknown ruling group prior to that date, succeeded by new rulers centred further North, who can be identified with the Brigantes. After that things seem to have stayed fairly stable, with the proto-Brigantes extending their rule over a large part of the region until we arrive at the situation of Cartimandua vs. Venutius in Roman times. In the South, things seem to have been less stable. Most of the major centres are much younger. Aylesford and Swarling have been mentioned and opinions are always changing, but the tendency seems to be to move dates later: where once there was a conjectural "Belgic Empire" in the Southeast opinion now seems to be shifting towards the view that changes in material culture are largely due to direct Roman influence from Gaul. In EBI's version of history, Caesar's Cassi are associated (per Ó hÓgain) with the Gaulish tribes whose names ended in -casses and made a major power, but that's only one man's view in the end. What was really going on there is anyone's guess...