Proof testing is not that expensive, just over £20. What does cost the money though is the preparation as bores must be polished out so there are no marks or dents and bulges and it is tight on the face. If the barrels are clean then this cuts it down a lot but then there will be the shipping. I would say that it is worth it if the gun is considered o.k. by someone knowledgeable about English gun proof requirements and the barrels walls are thick enough. I have submitted many damascus blackpowder proof guns for nitro proof and the vast majority have passed. Bonehills were usually strongly built guns. Lagopus.....
