|Beakers in the world|
If I understand correctly, Koch theorizes that Proto-Celtic is partially defined by the kind of errors that a speaker of Proto-Iberian or Proto-Basque* would make when learning Indo-European. Both languages had similar reduced consonantal systems (p-less) theoretically making IE sound Celtic.
In "Celtic from the West", Cunliffe and Koch theorize that Proto-Celtic was refined in Atlantic Europe in the Middle Bronze Age rather than being an import by warriors riding unicorns from Central Europe.
Atlantic Celtic has several strong arguments
1) It is demonstrable that languages spoken in Spain and France, regardless of their inter-relation, had common reduced consonantal systems in their ancient forms and that Celtic happens to be reduced from Indo-European in similar ways.
2) On the same token, the sound changes that made Celtic "Celtic" cannot equally be explained on a theoretical basis by any known substrate in Central Europe, that is if we were to assume external factors.
As mentioned on Genetics (Page 1), we have upper and lower limits for what is possible in the development of Late Western Europe. It's like burning a candle at both ends.
Linguistically, there aren't a whole lot of plausible options as to what languages Bell Beaker people spoke. There's really three plausible scenarios:
1 Beakers spoke Vasconic but eventually changed to IE. The map above doesn't help this scenario.
2 Beakers spoke a mixture of Indo-European and Afro-Asiatic. If Beakers came from the African Steppe, and particularly Southern Algeria and Morocco, then their IE may have already had a slight Afro-Asiatic substrate. [here]
3 Beakers spoke the very basal Italic and Celtic languages, both being sufficiently detached from further developments/refinements by seclusion in the African steppe, post SPR.