A double EEb is effectively that; two instruments in one with a 4th valve. The 4th valve allows the "gap" to be filled in below written F# but more importantly to give alternative fingerings for those out of tune notes such as written C#.
Very few would use a single instrument now, with the...
JP Sterling is a mass produced model. Paul Riggett makes his Sterling by hand. Check out:
John Packer Ltd work with Sterling so can organise something if Bedford is too far to go.