Celtic religion - the worship of Lugh, Epona, Mapon, Coccidius etc etc - is quite distinct from the cult of Druidism.

The former can be proven to have existed in Britain and Ireland through surviving placenames which commemorate the God or Goddess.

The Druidic cult (there IS proof for its existence in Gaul) has left no trace at all behind in either Britain or Ireland. Ergo, as far as we know, there was no Druidism in these places.

The meaningless equivalence of "Celt=Druid" is just baseless, and a fantasy of modern New Agery, I am sorry to say
PL