Several thoughts come to mind, the first being that while Cerrakote may have its' place on certain guns, a vintage English SxS isn't one of them. OK for a polymer frame semi, or a barn gun. Certainly fine for a cheap pump shotgun subject to harsh conditions on a boat or sea shore. Most people just aren't into painted doubles, and would probably prefer buying one with well worn bluing rather than one that is Cerrakoted.
I understand the reluctance to pay big money to properly rust blue a gun that may not be worth a lot. And while rust bluing is cheap and fairly easy to do yourself, there is a learning curve to do it well.
Another thought is what sort of oven do you have that will accommodate the controlled heating of say 28" or 30" barrels? I'd think barrels would need to be suspended so they weren't touching oven walls, racks, etc. Even if they would fit in a conventional kitchen oven, I'd think most wives would frown on using it to bake Cerrakote. I do agree that the baking would be fine so long as you stay safely below the melting point of the solder.
Finally, have you considered Oxpho Blue? It is by far the best cold blue I have ever tried, and cheaper than Cerrakote. Done well, it would look far better on an old English double. It would also be far easier to remove in the future. I bought an F Grade Lefever parts gun that had been Cerrakoted, and it was quite difficult to remove, especially in places like lettering, engraving, and along the ribs. I used methylene chloride paint stripper, lacquer thinner, and lots of steel wool and elbow grease.
Oxpho blue is very durable, unlike most other cold blues, and with a little practice, can be put on very evenly. I find it works best to apply the solution with small wads of clean 0000 steel wool instead of cotton pads. I burnish it into any places that it takes unevenly. I only used it on one complete gun, an old single shot, and was very pleased with how it turned out.
