Trans-Channel trade was an important part of the Gaulish trade network since centuries, trough the "tin roads" using Loire, Seine and critically Rhone as the main tradeway in the region; but since the IIIrd centuries, North-Western Gaulish and Southern British peoples shared a lot of similar cultural features; probably to be tied up with Belgian peoples being present on both sides of the Channel.It's not really clear if it happened trough conquest, migrations, establishment of familial ties or sheer trade links (although outright conquest is not this much considered nowadays due to the lack of strong evidence) but there peoples considered as Belgians in the broadest sense on both banks (between Loire/Somme and Rhine depending who you asked; and in most of southern Britain). It's hard to determine what made a Belgian people (probably not language giving the difference with Gaulish was at most a matter of more conservative speech) but peoples in Britain shared name with continental peoples (Belgae, Atrebates, Parisii), British onomastic looks really close to Gaulish (rather than Goidelic) and druidism was attested in Britain tough (that being said, possibly carried from Gaul).Even the absence of oligarchic regimes, with more systematical presence of royal leadership, or the absence of oppidae until the Ist century AD (maybe as a reaction, both economic and politic, to the Roman conquest of Gaul) can be partially found in some remote Belgian peoples in Gaul.
It doesn't mean that cultural make up of Britain was suddenly changed : in fact, there's more evidence for continuity from the Early Iron Age than not, the "Arras culture" in Yorkshire being an exception, with a regional melting-pot material culture emerging eventually, with influence varying in importance from Belgian Gaulish peoples in Armorica or Belgica; which dominated the area trading with Britain (especially at Hengistbury Head), and the raids recorded by Caesar, if happening, didn't let a great mark. Amusingly, the most import changes were due to Roman conquest of Transalpina, which increased trade trough Gaul and the familiarity of Brittons with Gaulish goods as well as the reverse; hinted at the increase of Belgian coins in Britain.
Altough Suessiones claimed having a strong influence in Britain, it's impossible to really spot real political alliances behind a sense of kinship (Commios fleeing to Britain to join his kind, as recorded by Caesar) and trade partnership. There's only one mention of Brittons joining Gauls in the war; namely Armorici led by Veneti, who are said by Caesar to excel in naval trade with the northern island.
[Veneti] unite to themselves as allies for that war, the Osismii, the Lexovii, the Namnetes, the Ambiliati, the Morini, the Diablintes, and the Menapii; and send for auxiliaries from Britain, which is situated over against those regions.
The problem for Caesar was less that Brittons mercenaries were raised by Gauls, that Gauls were able to supply themselves in Britain. Which giving on Caesar relying on requisitioning Gaulish products and grain at his benefit, was a logistical issue.
During the short part of summer which remained, Caesar, although in these countries, as all Gaul lies toward the north, the winters are early, nevertheless resolved to proceed into Britain, because he discovered that in almost all the wars with the Gauls succors had been furnished to our enemy from that country.
Now the problem is that we know significantly less about Iron Age Britain than Gaul: less texts, less archeological evidence especially in the South-East... But Brittons don't seem to have considered Gaul their bulkwark against Romans : while political changes (especially tentative to obtain regional hegemonies from Atrebates and Trinovantes) might be tied up with Roman conquest of Gaul, and even if Caesarian campaigns in Britain was partly made due to the supply of Gauls against Caesar, it's more a matter of trade and good neighbourhood.
