I predict that the longer we here in the U.S. go without a regular supply of 2 1/2" cartridges that people will forgo the need to maintain British Proof and lengthen the chambers in their English guns.
After all the US has no proof laws. As long as wall thickness is maintained to a proper safe level then there should be no problem.
It seems every time I sold an English gun one of the very first questions were, "Is it still in proof".

Mike Proctor