So I've been thinking alot recently about proto indo-Europeans and how they are the common Ancestors for most Western European Cultures. Also about how much of what we might not know about ancient cultures such as Germanic, Celtic, and maybe even proto Slavic cultures because they didn't write anything down and the only sources we have are foreign and biased.
So with that in mind how different were these cultures that we think of today when we talk about, let's say iron age Celts in Gaul and Rhineland Germanic cultures? Where much more closely related than we might think or where they much more diverse. At what point would these cultures have become distinct?
Another thing that comes to mind with this is that were brittonic cultures very much alike with Celtic cultures on the mainland or where they something different?
Sorry if this just sounds like a load of rambling
For what matter the difference between Gauls and Germans before the Roman conquest,this answer might interest you.
Long story short, the difference between Gaul and southern Germania was mostly a matter of regional identification and growing social and institutional differentiation; while both regions partook to the same archeological ensemble (Hallstatt and La Tène), with "Germanness" being about a geographical origin and institutional delimitation more than linguistic (with both Celtic and Germanic speakers being present in this "boundless" region, often forming coalitions).
The Rhineland before Caesarian conquest was certainly culturally diverse (although with a probable predominance of "Celtic" features) but with important commercial and political links (Sequanii and Treviri making themselves specialists allying and sponsoring migrant groups in Gaul). The name Germania itself might be a Latin translation of a Gaulish name for labeling a region inhabited by "kin" people.
Language was not really relevant in the institutional make-up, eventually, even if the pressure of Germanic-speaking peoples in southern Germania, pushing other peoples to migrate in Gaul, certainly colored the Gaulish and Roman perception of the region beyond the Rhine as "foreign".
The essentiallization of Gaul and Germania as different is a consequence of Roman conquest : the conquest of Gaul led to the decline or collapse of southern German peoples due to severed ties with Gaul, and a seemingly recomposition of regional identities along Germanic lines, even if La Tenian and Celtic elements didn't entirely disappeared. Eventually, the Roman classification of Gauls as "less barbaric" (i.e. assimilable, worthwhile) and Germans as "more barbaric" (i.e; worthless) especially after the Roman failure to conquer the land beyond the Rhine both reinforced this sense of distinct identity among Germans, but also the narrative and ideological fortune of the utter distinction between Gaul and Germania, finally enshrined by nationalist use of historiography in the XIXth century.
(Partly taking from this answer)
We could argue that the relation between southern Britons and Gauls was somewhat comparable to the one between the latter and Germans.
Leaving aside the historiographic debates about the protohistoric settlement and development of Bronze Age/Iron Age Britain, the isle seems to have been fairly peripheral to the archeological cultures of Hallstatt and La Tène, while ancient Britons unmistakably spoke a Celtic language. Being "fairly peripheral", however, do not mean being isolated : since the Bronze Age at least (e.g. the "Wessex Culture"), Britain was receiving a fair lot of mainland influences during the Iron Age trough the North Sea and particularly the Channel.
These influences probably took the form of trade exchanges but also migrations, without exclusivity (and probably reinforcing each other). While the nature of the exchanges isn't really debated, the question of migration is still is and not lightly : evidence for both their existence and nonexistence isn't definitive and sort of a compromise idea might emerges currently, with settlers being potentially everything from migrating iron prospectors to whole migrant people fleeing Belgian pressure in Gaul, including mercenaries or adventurers.
Historical evidence for migrations themselves are attested tardily, trough Roman sources : both Caesar and Tacitus account that "maritime Britain" (i.e. southern and eastern Britain) was contrary to the rest of the island settled by people from the mainland and especially Belgians which would be corroborated with the common onomastics (Parisii, Atrebates, Belgae, etc.), and possibly a sense of kinship across the Channel (with Suessiones claiming some sort of influence on Britain, or Commios gaining a kingship in exile in the island).
Archeologically, it is hard to tell the difference between result of migration (permanent or temporary), influence or exchange (especially as both weren't exclusive and could even reinforce each other) but the general idea is that while mainland influences are observable in Britain, they co-existed with maintained indigenous production. Even the distinct Arras Culture (characterized by its use of mainland style chariot or square-enclosure burials) didn't as much removed than completed local momentum : pottery remained fairly local, basic production such as housing (round in Britain, square in Gaul) remains fairly distinct...
So, when Caesar (DBG V, ) or Strabo (IV, 2) argues that Britons were similar, sharing a same lifestyle at the exception Britons being more barbarians (in the same model than Germans supposedly were), were they wrong or exaggerating?
Not entirely : some elements would support the idea of a relatively strong connection between Gaul and Britain, such as an important linguistic proximity between Gaulish and British largely considered belonging to a sub-branch; formation of southern Gaulish oppida would owes both the the Roman influence in the island but also to the Gaulish model; the very important trade relations between north-western Gaulish peoples and southern British peoples (Roman wine finding its way to Britain, British grain being supplied to Gauls during the Gallic Wars) in a Channel that Camille Jullian nicknamed "a Belgian Mediterranean Sea". Lastly, druidic presence in the island was important enough in the Ist century BCE that Caesar could suggest, possibly wrongly, that their doctrines originated from there.
Eventually, as with Germans but for different reasons, it's hard to consider the proximity between Gauls and their neighbours as a simple slide from "close" to "unrelated" : trade connections, sense of kinship (not as "Celts" but as the result of people-to-people relations, mostly Belgians), political ties, even religion or philosophy connected both regions. But Britain wasn't only the "recieving end" of these mainland influence but integrated them actively into an indigenous development : it's pretty much unclear at best how it was done, but it is clear archeologically that distinct features due to either geographic isolation, less developed social-economical structures (except in the latter part of the Ist century BCE) Britain didn't fully partake in the same ensemble Gauls and southern Germans seem to have, not at the same scale.