I don't think it matters what the members of this Forum think.
The way I understand British Proof Law is that the Proof House will inspect a firearm and determine whether it can be Proofed or Re-Proofed and what they determine is Law.
If the Proof House is telling you that removing metal from theforcing cone takes the firearm out of proof, then I don't know what recourse you have under British Law.
Knowing or not knowing whether something was done is not the issue, it's whether firearm is considered in Proof(safe to my understanding).
Of course in the USA, British Proof Law does not have any Legal bearing.
From an Engineering perspective, at least my Engineering, anytime you alter the pressure retaining components of a firearm, you alter the design and call into question whether the firearm is safe.-Dick
