Ratchford has 1.5mm greyboards that are more or less the same as Davey --
You might also try J. Hewit & Sons (). So far I haven't found any UK suppliers who explicitly state that they carry Davey board, but greyboard/millboard is pretty much what you want.
That's a pretty nice first book, too. I gave it a +1 on Google+. You should also tag these photos with hashtag #bookbinding so they'll be more visible.