having done a bit of reading on archaeological work in the area, it seems that the Middle Pre-Roman Iron Age (corresponding to La Tene B-C mainly) differs drastically in Britain from the Late Pre-Roman Iron Age (late LT C and LT D, roughly 150 BC on), with the latter period bringing the introduction of coinage, continental or Belgic pottery, larger and more developed oppida, and clearer signs of an organized, stratified society.

Now, the traditional interpretation has been to see in this the mid-2c arrival in SE England of Belgae tribes, and thus a shift in the social structure and material culture of SE Britain. It has also been argued that the changes seen in the mid-2c reflect the culmination of changes that began c.300 BC. In the 3c, previously undeveloped areas of "marginal" Britain began seeing greater development, and in their use of hillfort-oppida and more advanced use of land, the inhabitants of those lands demonstrate the ingenuity, organization, and precautions of large-scale successful pioneers, according to some interpretations. The slow emergence of social strata, signified by the very slow emergence of fibulae, and the development of both warrior burials or devoted weapons in the "marginal" zones from the 3c onwards, and chieftain burials in the SE oppida in the 1c bc, may indicate that the mid 2c event marks the culmination of developments that really began in the 3c.

The question then is how you want to interpret the various types of expansion. You see, in the 3c in the Midlands and such, this innovative expansionism. Because of the lack of evidence for a highly stratified society in the central areas, its hard to posit an organized project of expansion into the margins. But it does seem that the expansion projects have a reflexive effect on the older centers, at which places--by virtue of internal developments or by the arrival of Belgic invaders--some really powerfully evident changes take place relatively quickly in the middle and late 2c bc.

Also, Yorkshire, corresponding I suppose to the Brigantes, seems to be the place with the greatest evidence of La Tene culture: swords, torcs, inhumation ritual, metalworking of various sorts, etc, prior to the explosion of continental materials and oppida in the mid 2c.

And I haven't read it, but it sounds like Cunliffe's Iron Age Communities in Britain would be a useful source.