Do 20% really use e-readers? Sounds like a huge number. Do 20% even read frequently?
I'm not from the UK, and I'll admit my current reading habits are a sad excuse for what they were in my childhood and teens, so there might be some selection bias here.
Probably "20% of readers..."
I can say that out of the people I saw reading during my last beach vacation, almost all were using an e-Ink reader. They used to keep a small library of "beach books" you could read; that's gone. Casual readers have moved on to something else; people who read a lot have moved to e-Ink for outdoors.