Ask the insurance company what they'd accept. Are you talking about shooting insurance or theft/damage insurance?
BASC insurance doesn't ask for proof but if you were involved in an incident and it was found to be over 12 ftlb you just probably wouldn't be insured I suspect.
The only result a court would accept would be a forensic laboratory because their equipment is professionally calibrated and tested regularly........but that would be expensive.
.22 Webley Vulcan; .177 BSA MS Hornet; .177 FX Cyclone; 4 x .177 Gamo Delta's; .177 Brocock Hunter thumbhole stock; .177 Brocock Hunter Sporter.