I thought there was a law in Great Britain wherein a gun cannot be sold unless it is proofed (proven). I also thought this especially applied to guns that are imported.

Obviously there is a crack in this thought as we see guns with only BP proof marks being sold here every day and the onus in regards to shooting nitro loads lies with the owner.

What exactly is the straight dope on the British mandates regarding this stew of gun proofs, sales, importing etc?