As far as I understand it (and I suppose I'll have to follow Malcolm
Chapman's argument about it) the reason why the Celtic Church was
seen as different was because it developed from early medieval
christianity with little interuption whereas in England Saxon culture
disrupted things for a bit. By the time things evened out at the
Synod of Whitby, enough time had passed so that what came to be
called the Celtic Church appeared backwards and heretical to the now
dominant, now christian Anglo Saxon society.
Let me know if I've got the argument wrong.