Ah yeah.. you see it comes down to what's 'Reasonable' in law.
Now if the court accept that putting a couple of drops of 3 in 1 oil in the port is 'Reasonable' then you can argue that if you do that to any springer in the UK today it will put it over power.
And then you can argue that the intention of the law wasn't to make all airguns illegal, so that test isn't 'reasonable' nor was it the intention of the law.
It's the same thing with AT.
If you have an AT gun that's shooting under the limit with all the reasonable tests you can do (trying different weight pellets etc). Then a Forensic lab is on very shaky ground if they decide to swap components out in order to make it exceed 12fpe.
I'd call the Birmingham Proof house as my expert witness and also cite the above example that it wasn't the intention of the law to make every airgun in the UK illegal, therefore the test isn't a reasonable one, nor is it in the spirit of the law.