You can order British versions from amazon.uk. I got paperback versions of the first four from a local used bookstore that imports a few copies. None of the major bookstores are allowed to carry them.
Personally I find the American illustrations to be quite ignorable.
I've just been at amazon.uk. There's a 4-volume paperback set (adult edition) that I'm considering. I wonder if I should wait for the 5-volume set? I wouldn't mind hardbound, but I'm wondering if the shipping cost would be more.
Does the adult version mean no illustrations at all? Or just ones better suited for adults? I noticed a dragon on the side of the boxed set.