Was there a Kingdom of Britain before Caesar?

[Gaius Sempronius Gracchus]

Cassivelaunos was the king of the Cassivelauni, but he could be considered the "High King" of southern Britain. The chieftains of the Cantii, the tribe who faced Caesar first, went to ask for his aid. With Cassivelaunos came also the Atrebates and the Trinovantes.
Another proof of his authority can be found in his order sent to the four Cantii chieftains to attack the roman base camp, in order to relieve pressure on the Cassivelauni, so that they could reorganize.
Commios was the intended pro-roman ruler for the Atrebates by Caesar, also he acted as ambassador to Cassivelaunos.

Indeed, even within Caesar's own recount there are hints as to the true nature of the politics of Britain at this time, but he is at some pains to describe Britain as a set of independent tribes - I'm just wondering how much this was the case prior to Caesar's involvement.

There are references within the Gallic Wars to previous Kingships among the tribes of Gaul, and indeed sovereignty by one tribe over others and that got me thinking about how these areas were manipulated politically by Rome. Why, for instance, after Bituitus was defeated by Quintus Fabius Maximus were the Arverni - unlike their allies the Allobroges - allowed their independence? It seems to me that they were utilised to undermine the Aedui, who were becoming a force within Gaul. What the Romans didn't want was to have a unified Gallic 'state' over the Alps, and without the Arverni interfering and warring with them, there was a possibility that the Aedui might gain suzerainty over all of Gaul (well, if one ignores the tribes known as the Belgae, of course).

The Romans recognised this by treating the Aedui (on the surface) as "friends and kinsmen of Rome", but they seemed quite happy, in fact, to let the Aedui be battered from all sides (making treaties with the Sweboz, for instance, instead of coming to their aid).

Its too easy to perceive the Aedui as pro-Roman, and the Arverni as anti-Roman because of these positions, and the rise to power later of the Arverni noble we know as Vercingetorix, but that ignores that Vercingetorix was, essentially, a renegade of the Arverni - he had to overthrow the 'legal' Arverni leadership in order to campaign against Caesar.

What is the point of all of this rambling? Well, that the Romans looked to use factional instability both between and within the Celtic tribes in order to strengthen their position (or rather, to weaken the opponent - divide and conquer), and that I don't believe that this is a policy that was designed around the Gauls/Celts simply because of some perceived ethno-cultural weakness on their part (though there was certainly an element of this), but rather that this displays the nature of the internal politics of Rome as much as it demonstrates their external policies.

So, back to the original missive; we see that consequent to Caesar's brief campaigns in Britain (military campaigns, at least) there was an attempt by the Catuvellauni to re-consolidate their sovereignty, rather than what is normally perceived as an attempt at sovereignty simply after the event.

In other words, without the political and brief military interventions of Caesar at this time, the later invasion of Britain might have been a very different prospect.

[Arjos]

About the pro-Roman and anti-Roman, in every tribe there were "parties" divided. For example Dumnorix and Divitiacos.
The Allobroges it is possible that they were "controlled" due to their territory sharing border with the expanding republic. I remember that around 63 BC they complained for maladministration.
The Arverni saw their "kingdom" and alliances fall apart, having a pro-Roman oligarchy maybe was enough to keep the divide et impera...
For the invasion during the principate, the situation was quite similar: Caratacos expanded against the Atrebates/Cassivelaunos against the Trinovantes; same "High King" authority.

[Gaius Sempronius Gracchus]

About the pro-Roman and anti-Roman, in every tribe there were "parties" divided. For example Dumnorix and Divitiacos.

Absolutely, that is exactly my point. That one cannot simply judge the Gallic tribes as single, homogeneous entities; that they were, within themselves, divided polities, a situation well understood by the Romans, as much because of their own internal politics as well as from their allies within the various Gallic polities.

The Allobroges it is possible that they were "controlled" due to their territory sharing border with the expanding republic. I remember that around 63 BC they complained for maladministration.
The Arverni saw their "kingdom" and alliances fall apart, having a pro-Roman oligarchy maybe was enough to keep the divide et impera...

The situation the Allobroges found themselves in is what one would have expected from the Romans, especially - as you say - given the proximity of their lands to Roman provinces. It is that the Arverni were allowed to keep their independence that, to me, is the puzzle. It is most un-Roman to allow a defeated polity their independence (without even so much as tribute(or hostages?), if we are to believe Caesar), and so that leads to the question, why? Why were the Arverni allowed their 'independence'? Or, to put it another way, were they independent in reality, or were they simply given the appearance of independence so as not to undermine their (the Romans) relations with the Aedui?

For the invasion during the principate, the situation was quite similar: Caratacos expanded against the Atrebates/Cassivelaunos against the Trinovantes; same "High King" authority.

Quite similar, but not the same. The Catuvellauni had to re-expand their sovereignty, which is a lot more expensive than simply consolidating and protecting it. There were (I believe) many Roman backed leaders within those territories that resisted the Catuvellauni (which can be seen in the number of them that went forth to plead with Roman Emperors, as well as that at least one of them had with them a Roman moneyer), and one can only wonder at the financial aid (at least) that they received in their resistance.

As I said, the later invasion of Britain was greatly assisted, imo, by the earlier machinations of Caesar..