So, to the north, the Dobunni, with Corinium (Cirencester) their civitas capital. It’s supposed that they spread down into Somerset as far as the Mendips. For Professor Cunliffe, judging by Iron Age pottery and coin finds, the River Axe was the estimated southern boundary (1).
In that case Bath would have been in their region (rather than the Belgae) at least during the Iron Age, and Professor Cunliffe suggested that there might have been a ‘southern splinter group’ at a later date (Ptolemy was writing c. 150AD), like the one around Ilchester for the Durotriges. Bath would then possibly have been the ‘sub-capital’ though there is no evidence for this.
So far, so … probable/possible.
To the south west were the Dumnonii in Cornwall, Devon and west Somerset, with their civitas capital Isca Dumnoniorum (Exeter). Their northern boundary seems to have extended as far as the River Parrett.
To the south east are the Durotriges, civitas capital given as Dunium by Ptolemy which scholars seem happy to identify with Hod Hill, a hillfort about 4 miles north of Blandford Forum. Why this is preferred (if they were going to choose a hillfort) over Maiden Castle just a stone’s throw south west of Dorchester, I don’t know – perhaps there are more traces of sub-Roman civilian habitation; but Ilchester – Lendiniae – was certainly part of the Durotrigan territory in the 4th century and was the most north westerly centre. Ham Hill and South Cadbury are both described as ‘Durotrigan’ at some point. Hengistbury Head on the south coast, where the Stour and the Avon reach the sea at Christchurch, was an important trading centre with a mint and is another suggested site tribal centre.
This leaves the Belgae. Using circles to show the (very rough) possible settlements of the four tribal groups, as discussed:
We know where the River Parrett was, we know where Ilchester/Lendeniae was, we know where the Mendips and River Axe were. This leaves a kind of No Man’s Land in the centre. Did the Durotriges – or the Belgae – get to the Somerset coast in the 1st-2nd century AD? Ptolemy thought the Belgae. At any rate, that No Man’s Land is worth looking at next.
(1).B.Cunliffe, Iron Age Communities in Britain: an account of England, Scotland and Wales from the seventh century BC to the Roman conquest 3rd edn (1991), pp.170-5. Referenced on a website about Saxon Bath.