What the Romans didn't realise, was that the King was only a consort. That the Queen was the real power in those Tribes. And that just because the King died with no sons, the Kingdom was up for grabs. And that not all the British Tribes were of Celtic descent. The Roman 'conquest' of Britain was never as cut and dried as the History books would have us believe. Up until that time, he conquest was as much a cultural one, as a Military one. Remember, Rome had been pushing westwards for centuries by the time they came to Britain. Britain was where many of the Gaulish/Celtic peoples who opposed Rome, had been for a long time. The Iceni were one of the first Tribes the Romans had 'conquered', but they did it by supplication, not Military force. With trade, and Gold, and Wine, and access to all the things the Romans imported from every corner of the Empire. Over the years, this made the Iceni powerful and influential among the other Tribes. And we didn't have a National Identity as such, we didn't identify as British, our affiliations were Tribal, not National. There was no High King, or unity beyond Tribal Chieftains. Certain Tribes had alliances with other Tribes, but they all had very different and separate identities, customs, and even Gods.
So when the Romans, with their Imperial mindset got here, they saw Britain as a Kingdom, without an overall Ruler.
Full of what they saw as squabbling Tribes, who could be worked on, individually, until Roman influence stretched from the East coast, to the Severn, from Cornwall, to Scotland. Then the Generals could return to Rome, with the Laurels of success, having expanded the Pax Romanus throughout another Kingdom.
And they kind of stretched themselves a bit thin in Britain. To the North were the Picts. Hadrian's wall was the worst posting a Legion could ever hope to get, in the whole of the Empire. To the West, were the wild Welsh Mountains, where the unconquered Ordovicci slaughtered any Legion that strayed into their Mountains. And across the Irish sea were, well, the Irish. Any envoy to Ireland had to be on their very best behaviour, or they never made it back again. They traded with the Irish, for gold, and dogs, but never made any serious Military forays there.
But the only thing that all these disparate British factions had in common was the Druids. Druids moved freely between the Tribes, carrying news of Roman activity from Kingdom to Kingdom, stirring up dissent and resistance. So Caesar decreed that no Druid should set foot in Britain on pain of Death. (A Law that was only technically repealed a year or two ago)
They were eventually pushed back to the North of Wales, and their stronghold on Anglesey. But their influence could not be banished, because all the Tribes would shelter them, and protect them. So that's when Gaius Suetonius Paulinus took all the Legions he had, up to Anglesey, to put down this nest of Vipers once and for all.
Leaving the 'subdued and Romanised' Iceni and Trinovantes to rise up, under Boudicca, and almost take back the whole of Britain from Rome. It was only bad luck and over eagerness that lost the day for the Tribes. They were so sure of victory, after sacking London and Colchester, that all the Women and Children turned up to see Paulinus' and his worn out Legions arrive to the slaughter after their forced march back from Anglesy. But this left the Tribes hampered by the fact they couldn't make the tactical retreat that would have drawn the Romans into a pincer trap. So Rome won the day. Just. But it cost them dearly.
And they had to revise the way they dealt with the British in future.
After that, Britain was just the farthest outpost of the Empire. And it's Military influence dwindled as the Legions were drawn back to deal with the Visigoths threatening Rome from the East. The Legions stationed in Britain were largely from other far flung parts of the Empire, and as Rome's power decreased, they went Native, and settled here, and began a more equal Romano British culture that lasted right up until the Saxon incursions of the 8th and 9th Centuries. Centuries after Rome had collapsed, there was even a Romano British expeditionary force sent to try and take Rome back from the Goths who had overrun the Empire. So technically, the last Roman Legion ever was made up of Romanised British, who centuries later, felt strongly enough about Rome to try to regain control of the city, long after she had been sacked by every marauding Tribe of Goths, Visigoths and Huns who wanted a piece of it's faded glory.
Any Druids who survived Anglesey, went to Ireland, and were eventually Christianised. And written out of any subsequent Histories, and Demonised as devils from the Pagan past. Caesar's surviving campaign diaries were the last surviving first hand written accounts of Druids. All other accounts were 'Romanticised' and told as legends, and stories, and so they passed into myth, until the 17th century revivalists started to take an interest in our past. But by then, there was very little for them to go on. So they made so much stuff up, in the style of Geoffrey of Monmouth, and Sir Thomas Mallory, that any credibility is dubious to say the least.