According to what I've read, minimum resale price maintenance, aka agency pricing, is illegal in the United Kingdom.
Every book I've ever looked at on Waterstone's site is priced higher than the US agency pricing anyway, so the whole question may be academic.
They are having a rough go of it for sure.
I know they tend to run high, but it's the only place you can purchase some books. Probably having to do with digital rights flights in the US between the author and publisher.