I know the one in A'en and there is supposed to be one in Brussels.
There's also Carron in Gent (Veldstraat). It's basically a tobacco shop but they also sell expensive gentlemen's stuff like straight razors.
I think I wrote this earlier in the thread but also check out theinvisibleedge and the srp classifieds section. There may be other mail order cies in the uk but none that I know of.