Sorry buddy, this is just wrong
The law says "capable", it says nothing about the police proving you intentional made it so.
As an example, I unknowingly buy an air gun running at 20fpe from a bloke in the pub, I've never altered or tested it, and he says it's only 6 months old and a standard sub 12fpe gun - if it gets tested a week later while in my possession - will the police do me for having a firearm
The answer is clearly YES