I think Miriam Margoyles would be brilliant for Nanny,she´s just the comfortable,blustery type with a twinkle in her eye like you would expect.
I like Maggie Smith for Granny,she has this air of indesputable command,which was why she was brilliant for Prof.McGonagall.Besides,I think those two would enjoy playing Pterry so much,that it would bring a lot of esprit to the dialoges.
I´m not sure about Magrat,I think so far I think Alison Hannigan is the best vote.
For Verence,if we are looking in the Harry Potter cast,I´d prefer one of the Weasley twins.They are experts at being funny,so would be a ggod choice,but can also play serious parts,too.Best is,they can switch from funny to serious really quickly.Rupert Grint always seemed to be the eternal idiot to me,much as I like him in HP.
It´s still magic even if you know how it´s done.