Merlin used to sell a TDB frame for £135 and a WCS frame for £200. The WCS was obviously made of Ritchey Logic WCS and the TDB stood for Tange Double Butted. In the late 90s they quoted it as 'the latest Tange tubing with spiral butting'. The TDB had wishbone seatstays, Ritchey dropouts and ti plated finish, so it doesn't look as though this particular bike is a Merlin.
Whether the TDB was made specifically for them I don't know. It seems just as likely at that price that it could have been sold to a number of outlets in various countries, with decals applied locally. I doubt whether it was made by Tange though. There are several huge factories in Taiwan that build frames in the kind of volume that can support that kind of price structure. I thought Merlin frames were made in Fairly, at least some of them.
There's a lot of snobbery about shop brands, but a Tange double-butted frame is a decent frame, arguably higher quality than a mid-range frame made of generic tubing like a Rockhopper or a Cinder Cone.
