Let's assume the Romans decide the British Isles aren't worth taking; it's simply too far away and cloudy for civilized hands to dirty themselves. Or failing that, Boudicca succeeds and scares Nero off the island. Got a third idea? That works too, I'm just throwing stuff out there to keep the Romans away. If we can make this happen, I'd like you to answer a few questions with your best educated theorizing:
1. What happens to the British Isles? Do they develop on their own and create polities based on Celtic culture? Do the Germans possibly use it as a funnel for their limited territorial expansions by the Roman Empire? Do the Picts have a shot at survival?
2. What happens to the Roman Empire? Is their power or wealth damaged or weakened by a lack of presence in Britain? If Germania has the Isles to funnel to, does the ERE stick around for longer, considering there's less territory to cover and there's potentially less outside pressure for Germanic tribes?
3. Can the New World be discovered sooner? I'll admit I even find the idea a little ASB, but not sure if it's plausible. If British naval technology (more ocean based than Rome's sea based) continues to develop, the area is not dominated by a Mediterranean thinking power, and German immigration potentially shuffling the Britons around, is it possible some ships might sail north/north-west and eventually hit North America? I'm mostly just curious on this one because it seems very strange, but not totally impossible; I mean, there are legends of the welsh making it to America.
For a PoD, it is okay for Rome to continuously plunder Britain, just not conquer it.