We've discussed Roman Germany, and Roman Ireland, before, of course. So let's invert it.
Suppose Claudius never invades Britain? What becomes of the island? Well, in OTL the southeast was becoming increasingly Romanized, with the incipient urbanization we saw in Pre-Roman Gaul, increased coinage, tribal leaders minting coins with the word Rex on them, etc. So my guess is one of the tribes end up subduing the southern half of the island.
And then comes the 3rd century crisis. The King of Britannia is young, and charismatic. But the growing centralization of his state has caused unrest, and a few of those weird people, the Chrisitans, have started apearing. And he has visions of becoming not just a King, but an Emperor. And so, Artouiros sets sets sail across the Channel.
Okay, it's a vague musing while I study for finals, but I'm curious if people have thoughts.