It is true to say that the religion of Classical Greece was worship of the Olympian Pantheon.
It is NOT true to say that the religion of Classical Greece was the Eleusynian Mysteries.

Moreover, although we know for a fact that the Mysteries were practised at Athens, we do not therefore assume that they were practised at all other Greek city states. In fact, we doubt they were celebrated at many city states at all!

And Greece was far smaller than European "Celtia" (as it is termed here!)
To assume that what was done in Aquitania was done in Brigantia or Hibernia.... I can only ask "What is it in the modern mindset that NEEDS to make such assumptions?"