Genetics aside, I wonder if the lack of Celtic place names isn’t more a function of who was in charge when towns and cities were named?
I mean, plenty of rivers have names of Celtic origin, but it seems that most English town names are either Roman, Saxon or Dane in origin, probably because most of the towns were founded during those time periods.
There wasn’t a post-Roman, pre-Saxon time when the Celts ran around founding cities, and prior to the Romans, there weren’t really any major cities.