By British law to sell a shotgun in the UK it must be in proof. It's on the seller to make sure that it is. The auction houses take that seriously as it will drastically impact their livelihood. I haven't dealt with any auction in the UK that didn't or wouldn't provide wall thickness measurements if asked. I used to be a FFL/Import permit holder and did a lot of business buying from auctions. None have had walls below minimum thickness unless mentioned in the lot description and/or sold as action, stock and forearm only.
