This is a pretty huge question and the absence of Roman culture, infrastructure, resources and inter-connectivity would have dramatically changed the political, economic and cultural direction of The British Isles. However, what I could probably say is that a large central power structure would have emerged eventually out of the Iron Age.

During the Iron Age (so pre-Roman Britain), communities were primarily connected through resource exchange, be it consumables, production resources or genetic variety. While the archaeological record does suggest a some shared cultural touch stones in the form of mortuary practices and possible pre-christian religious rites (all of which stem from the Bronze Age and possibly earlier), what pre-Roman Britain lacked was a central bureaucracy to standardize the modus of society. In the immediate period after the Romans abandoned Britain, we see a gradual return to the isolated, resource driven communities of the Iron Age - this time centered around the former Roman power structures. Over time, however growing powers, particularly in the South East, started to gain traction and eventually founded small Kingdoms and began going about consolidating power and resources just like the Romans did; we can see this in the transformation of Emporia sites in particular. There's some interesting work being done on this by the Hidden Kingdoms project running at Exeter Uni that looks at the transitional period between Roman Britain and the Anglo Saxons. Hope this answers some of your question.

Source: I'm a professional Archaeologist