I few changes has been made to GPC since the expansion. This has raised some questions.

* Is Caercolun = Essex?
Looking at the new maps, this would put Essex further up north. If I go by the GPC maps, Thamesmouth would become Essex. Am I loosing my mind?
* When does Caerwent fall to the Saxons? 487?
* Count Roderick looses three hundreds when Cearcolun is taken, this is a major loss to even him. How does this affect him?
* What happens to Thamesmouth? Do the Saxons control part of it?

IMHO,

485 Saxon Invasion takes out Camulodunum and its environs, i.e. the southern half of Caercolun.
486 After the successful ambush of Count Lucius and his army, and the subsequent death and defeat of the same, Essex takes over the rest of Caercolun. The counterattack led by Prince Madoc pushes the Essex back around Camulodunum, i.e. the extent of Essex on the 485 map (limited between Chelmsford and Ipswich). So about half of Caercolun.

I'd say that they are not able to take Thamesmouth, but the eastern parts of that county would experience constant Saxon raids throughout Uther's reign, and during Anarchy, Essex would encroach on that land, too. Finally taking Londinium in 503, I think it was.

Caerwent lasts until 500, when Angles land and overrun it.

What effect this has on Count Roderick's lands depends where those lands actually are. Looking at the map in BotW, I'd say that they are in the northern part of Caercolun, and hence probably harassed from time to time by Saxon raids (and plundered in 486), but not conquered by them, until the Anarchy. Good possibility for some Saxon action if you feel the need for it for the Salisbury knights.