* Brythonic languages: It's likely that a common ancestor of the modern Celtic languages, called Proto-Brythonic, was spoken in Great Britain during the Iron Age. This language evolved into the Brythonic languages spoken in the Roman period, including Cumbric, Welsh, Cornish, and Breton.
* Goidelic languages: There's also evidence that Proto-Goidelic, the ancestor of Irish, Gaelic, and Manx, was spoken in parts of Great Britain during the Iron Age, particularly in Ireland and possibly parts of Scotland.
* Other possibilities: While it's less likely, some scholars suggest that other language families, such as Indo-European languages other than Celtic, or even non-Indo-European languages, may have been spoken in Britain during this period.
The evidence we have:
* Archaeological finds: Archaeological evidence, such as place names and inscriptions, points towards the presence of Brythonic languages in the south and west of Britain.
* Linguistic reconstructions: Linguists reconstruct the Proto-Celtic language and its branches, suggesting that Brythonic and Goidelic languages were spoken in Britain during the Iron Age.
* Later historical records: Historical records from the Roman period mention the presence of Celtic languages in Britain, further supporting the idea that these languages were spoken in the Iron Age.
Ultimately, we can't say with absolute certainty which languages were spoken in Iron Age Britain. But the evidence strongly suggests that Celtic languages, particularly Brythonic, played a dominant role.