I suppose if the Romans would have decided not to conquer the island , a local tribe became dominant and when the Romans later invaded, they might do so on a more limited basis....
The problem is, I believe roman pressure is needed to get tribes there to coalesce in the first place, at least in a form capable of withstanding an imperial assault. Sure enough, if the island is left alone some tribe might get ascendancy over the others and put together some kind of confederacy, but I don't believe they would get together the State institutions, the fiscal-military structures, needed to challenge Rome. In the best case, they would get something like Arminius' OTL germanic confederacy: tribes huddled together in a fragile alliance that might pull together enough forces to crush one roman army in a best-case scenario - but this would be their last mistake as the romans would immediately muster superior ressources to crush this resistance, as happened to Arminius.
OTL, it took the germanic peoples four centuries of near-constant roman pressure, with access to roman networks of trade, and contact with roman models, to develop consolidated and hierarchised proto-kingdoms capable of challenging Rome (granted Rome kept interfering to prevent the formation of such kingdoms, but this would happen to Britannia too anyway). Sassanid Persia did this more efficiently, after "only" the civil war that followed the Parthians' demise, but they were building on centuries of statecraft traditions and a vastly superior economic development level. And here we are hoping that a similar process of state-building and the constitution of an anti-roman mentality happens from a much lower level of development in a much shorter time, short enough for the Romans not to attract the Brits into their cultural and economical if not political orbit?
Of course, I'm following a simplistic linearist conception of statecraft here; in reality, state building isn't just progressing from tribes to chiefdoms to kingdoms into a full-fledged fiscal-military State following increasing levels of economic growth and institutional complexity allowing to support larger and more efficient armies. But still, there's something to that model. To beat the Romans at their own game, germanic peoples to some extent had to become like the Romans. They did
not simply unleash their primitive tribal energy on the decadent romans or something. And from my point of view, if we want our alt-brits to develop a "national" consciousness and the military power necessary to resist attempts at invasion or assimilation from the Continent, we need them to "ascend" in Statecraft. A bunch of tribes on an island just cannot have roman influence bounce away from them.
So the most likely possibility I see is... hypocrisy. As in, adopting a lot of Rome's tools and institutions while claiming these are perfectly indigenous and not-at-all influenced by the continent. Let's say for some reason the Roman attempts at conquering Britain under Claudius are even more half-hearted than OTL, allowing the Brits to start consolidating and strenghtening; an uprising/reconquest à la OTL Boudicca then starts a protracted war between Rome and the Brits, during which the latter develop an ideology about them being special for some reason all the while stealing all they can from the roman toolbox. At this point, we need to have the roman screwing up massively for some reason - maybe by having the war break out shortly before the IIIrd century crisis in the Empire prevents them from sending reinforcements? And voilà, an island state which hates the roman and lived to tell the story.
I recall the official conversion of Sweden being described in terms of "similar to joining the EU".
Where did you read that? I cannot fully measure the historical accuracy of the claim, but I can see some likeness: giving up a limited measure of political independence in some fields in return for a massive economic boost and hugely increased political influence in other fields. A good bargain either way. Maybe, maybe we just need to make our ATL Brits dumb?