It doesn't have to be.
At the moment we have everything you say plus the problem of the word "capable", they are all "capable" and therein lies the problem.
We have an operational tolerance on speeding limits and we all know speed kills. Doesn't stop ACPO from operating that tolerance though. 12FPE plus 10% seems reasonable. Bit like the speeding tolerance.
A standard simple test shouldn't be that hard, we're hardly talking about a weapon of mass destruction here.
It could be as simple as "the pellet found with the gun" and an obligation on us to ensure those pellets are available with the gun or something else would be used. I'm sure it would not be impossible to come up with a simple solution we can all operate.
The legislation pertaining to air rifles is just not fit for purpose (if it ever was). What's the problem with putting it right ?