Correct form of Iron Age British names

[Rumo]

He said "I don't think there is any consensus, just possibilities. Most of what we think of as Brythonic is primitive welsh back projected and most contemporary evidence is, as you point out, from romanised inscriptions"...

Now, while this may be true in some instances, it is not at all true in others. As I have already mentioned, there were many important Celtic inscriptions (and I mean fully Celtic and not "Romanized") discovered in the 19th and 20th centuries that vastly improved our knowledge of ancient Celtic dialects, so that we now speak with a degree certainty about many words. Additionally, Tacitus' ancient claim that the language of the Britons and the Gauls differed but little from one another seems to be supported by the onomastic evidence, not to mention the potentially Brittonic inscriptions published by RSO Tomlin.

And I believe he's correct in his general assessment. From my readings I also understood the relations between ancient Celtic languages to be rather controversial. For example: "the genetic subclassification of the Celtic languages is still an open matter" [see footnote about the two competing theories in "broad terms", one supported by Koch] and perhaps more important "Gaulish, Lepontic and Celtiberian are not attested well enough to clarify their relations to the Insular Celtic languages"

To answer your question, yes, I am aware of the mixed nature of the La Graufesenque inscriptions....what does that have to do with the Gaulish word for "four" recorded in them (which clearly shows no foreign influence).

If it was a multilingual community then almost every word may be suspected to be influenced by the other languages, and thus not a very reliable reference for a vast area (such as Gallo-Brittonic). Actually a localized attestation is hardly evidence for a larger region. Let me express my skepticism more clearly: can you prove the Welsh word for 'four' (or other similar more recent words) were not brought by other waves of Celtic speakers from the continent and were spoken in Iron Age Britain?