Perhaps he meant to say that druidism within Celtic societies evolved from northern Gaul and particularly in England... however this most likely based on the writing of Julius Caesar from the Fourth book of the Gallic Wars, XIII:
Particularily the last part - "This institution is supposed to have been devised in Britain, and to have been brought over from it into Gaul; and now those who desire to gain a more accurate knowledge of that system generally proceed thither for the purpose of studying it."
This doesn't prove that druidism originated there, all that it really says is that Britain was a primary focal point in that time. Then theres the extremely foggy matter of the Tuatha de Danann in early Irish myth, but it should only be understood in the context of myth.
If you want a far more solid starting point, look into the similarities to the Brahman in India. If you exclude the subjective stuff, most things you will find point to an indo-european origin for 'celtic' society and druidism.
But anyways, saying "Celtic culture evolved from northern Gaul and particularly in England. So don't say I don't know much about Celtic culture" is highly ironic and it makes me laugh. Go to the library, and read the texts that make use of real evidence. Then come back and and have a discussion that excludes generalities and subjectivity as much as possible. Thanks.
Oh yeah, use references.