Yeah, the ACW ships were a joke compared to the stuff the British could make.
Technology doesn't work that way anyway. You can't just sell 'tech' ala civilization.
And the confederate warships were for the most part entirely unsuitable for british requirements. Most notably, they were pretty much entirely designed for freshwater or litorial engagements, as they were built to defend southern ports, engage american vessels on inland waterways, and to break the US blockade. Not exactly the sort of ship that would appeal to a royal navy at the height of its power.
The submarine idea is also a non-starter. Leaving aside the issues involved (such as the
Hunley sinking on its first voyage), the craft was basically only useful against a blockade (not a position the british were likely to face), at best it could only threaten the preeminence of the british fleet proper, and thus would not be encouraged.
As you said, technology transfer is rarely such a simple process. In this case, the confederates dont really offer anything superior to the british model (at least, in british eyes), and their designs are made for a certain strategic and tactical situation which would not be envisioned by british officers.