Ask a few random folks of various ages, on the street. Check out YouTube views for some of their videos, weighted by decade (videos from the 80s with tens of millions of views can be considered as popular as stuff with high hundreds of millions now). Album sales. Chart rankings from multiple countries. Endorsement revenues.
He may have been a household name in England. I have no idea. If you're into electronic music, I think you're bound to see him mentioned somewhere because it seems people like namedropping him.
That said, I completely agree that you can't expect any young people to know who he is.
This is not rocket science.