Many many of his books were best-sellers, and remained so for a long time.

Plus I suspect a lot of parents had them around the house, and they've found a second life in their children.

The covers always fascinated my child, for example. I have the complete collection of Discworld novels as paperbacks, and I remember my kid looking at them, pulling them off the shelf to do so, even before he could read.

In short Pratchett was very much mainstream, in the UK for a long, long time.