Just as a minor point of information, Birmingham Proof House (I only use them, so can only speak from that experience, though I presume London do the same) do check head space as well as carrying out the Over Service Pressure test firing. Rifles do sometimes fail proof for excessive head space. As I know this to be the case I always check the space on all of my rifles that I submit, & I've had nothing fail in years, other than one rifle I submitted for someone else!