I don't know whether to enshrine this information on this permathread as it may change...but...
There are now apparently two suppliers of washboards in the UK:
Voss of Germany have an agent in Derek Mason at derek@skiffle65.freeserve.com and 147 Tudor Drive, Kingston on Thames, Surrey KT2 5NT
and Thrift music (info@thrift-music.com) at 21 Connaught Avenue, Frinton-on-Sea, ESSEX, CO13 9PN who are new UK agents of the US Columbus Washboard Co of "Ahia".
RtS (other sources linked in out-of-date pages may no longer be in operation)