Mary Poppins is marvelous. I loved it and saw it twice. Google the London Theatre Guide and see what's playing at the Royal Shakespeare Company, the Donmar Warehouse or the Old Vic. The Woman in Black is a definite consideration, and maybe Bloodbrothers.
There's always Mamma Mia, but my recommendation is to see things they might not get to see in the U.S. Of course, that would leave out Poppins. When I went in 2004, my friend liked The Mousetrap, but I went to see Dirty Blonde that night instead. If Journey's End is still playing, it's electrifying theatre,too.
NYC Visitor and Broadway Fan