The irony of course is that "Celtic" is a silly word to begin with. There's plenty of circumstantial evidence that the 'Celtic' culture of Gaul was reasonably similar (and quite close in cultural 'ancestry') to Roman culture anyway, making assimilation relatively painless. The British isles may have been a somewhat different case.
There’s no good reason to apply the word “Celtic” to the pre-Germanic inhabitants of the British Isles, other than some linguistic affinity. No ethnic affinity has been demonstrated. The word should be reserved for continental European cultures, as the Romans used it.
To be fair, "circumstantial evidence" in archaeology is just saying something because you found a pot that looks like another pot. There will plenty of "circumstantial evidence" to suggest the opposite. :)
The irony of course is that "Celtic" is a silly word to begin with. There's plenty of circumstantial evidence that the 'Celtic' culture of Gaul was reasonably similar (and quite close in cultural 'ancestry') to Roman culture anyway, making assimilation relatively painless. The British isles may have been a somewhat different case.