Some time ago I was discussing with our local Police Firearms Manager the possibility/problems of importing from the US, during the discussions I told him I was looking to re-stock one of my rifles.
A few days later I received a letter from he Firearms Manager informing me that I must now have the rifle (that I was re-stocking) re-proofed as changing the 'furniture' meant it was no longer in proof.
Several letters and a few weeks later he had not moved his position, so I wrote to the proof office asking for guidance, to which they replied (in writing) that they request that all firearms are submitted for proof without the 'furniture' to avoid any potential damage to the woodwork in their clamps.
I submitted the letter to the Firearms Manager suggesting that as I had now fitted the furniture was he proposing that I remove it, submit it for proof, then replace the furniture, at which time it would (according to him) need reproofing.
The matter seemed to go very quiet - I never received anything further on the subject.