Oddly enough, according to Caesar even axillaries from cisalpine Gaul (modern day Italy, but these people had much more in common with other Gauls than Italians linguistically and culturally until Roman assimilation) were able to understand most of the vernacular in Britain aside from idioms (he used Gauls who traded with the island for diplomatic interpretation to avoid a faux pas because of those difference)
Given that written Breton from the 800s were readable to Breton intelleculuals of the 1800s, this and Caesar's comment might mean that western Europe before the Romans were almost united in language.