Interesting question, John C, though I don't quite understand why you chose to ask it in this thread...

Anyway, a quick search would suggest that there's no simple answer to your question.

For example, according to this site Ilkeston was probably founded in the 6th century AD and gets its name from it supposed founder, Elch or Elcha who was an Anglian chieftan ("Elka's Tun" = Elka's Town).

Ilminster and Ilchester appear to have been named due to their proximity to the River Ile