With respect I think you are missing the point, or I am not making it strongly enough.

It's not a matter of something perchance taking the power over the limit, this is a component expressly designed for a higher limit. My question is, does that make it a part of a licensable firearm which is then subject to the relevant paperwork procedures?

Whether or not the owner has any duty of care etc isn't the question. I'm not making any moral judgement or any other comment on anyone selling such a spring, I'm just interested in debating the status of the component.

Buying a bronze bar that might be used to make a new component...... too remote. This is a spring that has just come out of a rifle that would be overpowered for the UK market. I hope the difference is clear.