on the dawn of the Roman Conquest, Britain was divided into a number of tribal nations (for lack of a better word), varying in size and complexity. Whilst it is an over-generalisation, the larger, more complex ones were in Southern Britain (particularly the South-east) and appear to have had a limited coin economy and had moved away from foritifed "hill-forts" as centres of power to proto-urban settlements known as 'oppida', which were enclosed by banks that seem more to be boundaries than defensive. Combining the archaeological evidence with the limited literary records, we asssume that these fairly large nations (often comprising several modern counties) were kingdoms. These areas were in regular contact with Rome and there is plenty of evidence of 2-way trade
The further north you go as well as into the west country, it seems the complexity and size of the tribal nations decreases as does evidence of Roman contact.
As to 500AD, ideas vary; to be honest I would recommend reading "Civitas to Kingdon" by KR Dark, it does deal with the issue and seems to give a good rundown of the Romano-British kingdoms, including the evidence for a surviving enclave centred on London.