The Coel of the geneologies was not "King of all Britons", he was simply a magnate whom some medieval Welsh nobles claimed as an ancestor. His date (5th Century) is too late to make him the ancestor of Constantine the Great. In theory he could have had some connection with the British pretender Constantine III, but I don't know any evidence for this.

Geoffrey's "history" is thought by modern scholars to be a satire, a sort of 1066 And All That. The issues Geoffry writes about would be those which were discussed seriously in his time (otherwise there would be no joke), but nothing he says about them can be taken as history.

That the town of Camalodunum is now called Colchester is a fact. Since "Colchester" is an English word on its surface, it's reasonable to suppose that the English gave the town its present name. That doesn't mean the town was named after Coel or anyone else. At some point in the middle ages, Carcasonne in southern Gaul was thought to have been named after one "Carcas"; this seems to have been a ficticious individual invented to account for a name whose derivation was no longer understood. Assuming, arguendo, that Camalodunum wasn't renamed after Coel Hen leaves us with two possibilities: Perhaps "Old King Cole" is a ficticious person, unconnected to the 5th century Coel Hen, who was invented to account for the town's name. Or perhaps Coel Hen left a reputation behind him among the English, and this reputation later led to the story that the town had been named after him.

T.