I thought that an English gun that was "still in proof" meant that it still had the dimensions it had when it was proof-tested. ie: bore gauge. If gun is stamped 13 on the barrels, then it was proofed at that bore diameter. If the gun now measures 12 gauge, it has been opened up to that larger bore diameter. This modification would mean it's no longer "in proof" and should be re-proofed (technically).
Now, one has no idea what the wall thickness was when the item was proofed, so like Bill said, it could have been struck/filed & you wouldn't know.
So the valid point is that proper barrel wall thickness is always a tangible measurement.
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