P-Celtic versus Q-Celtic

What is the highest probability alternative?

Did the innovation from Q-Celtic to P-Celtic sounds happen on the continent and spread to Britain in the form of Brytonic languages?

Or did the the innovation happen at least twice, once in Gaulish or pre-Gaulish languages and once on the British Isles?

Is this even a fair way to look at the same issue? Is Welsh closer related to Gaelic or to Gaulish languages?

Mike W