I'm not even sure whether The Birmingham Proof House is UKAS accredited for testing airguns.
What you would get in Court is the opportunity to present your testing evidence. The prosecution will have had testing done by a UKAS accredited test house. That evidence will be presented by their expert witnesses.
You can also have testing done by your accredited test house and that can be presented in Court. We now have two lots of expert witnesses arguing about the validity of their own testing procedure. We have this because there is no standard test and each lot of expert witnesses will be advocating that their test is the best.
Your on a 50/50 with the Magistrate at best.
If your pretty sure you rifle was shooting at "low" 11's then your probably not going near a Court anyway. If however it's shooting"high" 11's you know your sailing close to the wind and gambling on a test result.