Was there a Kingdom of Britain before Caesar?

[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..